Page 1 of 20 pages

                                  UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549

                                  SCHEDULE 13D
                    UNDER THE SECURITIES EXCHANGE ACT OF 1934
                                (AMENDMENT NO. 4)

                               HEXCEL CORPORATION
--------------------------------------------------------------------------------
                                (Name of Issuer)

                                  Common Stock
--------------------------------------------------------------------------------
                         (Title of Class of Securities)

                                   428290 10 0
--------------------------------------------------------------------------------
                                 (CUSIP Number)

    Robert J. Small                              Joel S. Beckman
    Berkshire Partners LLC                       Greenbriar Equity Group LLC
    One Boston Place                             555 Theodore Fremd Avenue
    Suite 3300                                   Suite A-201
    Boston, Massachusetts 02108                  Rye, NY 10580
    (617) 227-0050                               (914) 925-9600
--------------------------------------------------------------------------------
                  (Name, Address and Telephone Number of Person
                Authorized to Receive Notices and Communications)

                                 August 17, 2004
--------------------------------------------------------------------------------
             (Date of Event Which Requires Filing of This Statement)

      If the filing person has previously filed a statement on Schedule 13G to
report the acquisition which is the subject of this Schedule 13D, and is filing
this schedule because of Rule 13d-1(e), 13d-1(f) or 13d-1(g), check the
following box. [ ]

      Note. Schedules filed in paper format shall include a signed original and
five copies of the schedule, including all exhibits. See Rule 13d-7 for other
parties to whom copies are to be sent.

                         (Continued on following pages)



                                                              Page 2 of 20 pages

CUSIP NO. 428290 10 0

                                  SCHEDULE 13D

--------------------------------------------------------------------------------
1.    NAME OF REPORTING PERSONS
      S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSONS
      Berkshire Fund V, Limited Partnership
      04-3423237
--------------------------------------------------------------------------------
2.    CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*

                                                                     (a)[ ]

                                                                     (b)[X]
--------------------------------------------------------------------------------
3.    SEC USE ONLY
--------------------------------------------------------------------------------
4.    SOURCE OF FUNDS
        OO
--------------------------------------------------------------------------------

5.    CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS          [ ]
      IS REQUIRED PURSUANT TO ITEM 2(d) or 2(e)

--------------------------------------------------------------------------------
6.    CITIZENSHIP OR PLACE OF ORGANIZATION

            Massachusetts
--------------------------------------------------------------------------------
                  7.    SOLE VOTING POWER
NUMBER OF                 2,151,669
SHARES            --------------------------------------------------------------
BENEFICIALLY      8.    SHARED VOTING POWER
OWNED BY
EACH              --------------------------------------------------------------
REPORTING         9.    SOLE DISPOSITIVE POWER
PERSON                    2,151,669
 WITH             --------------------------------------------------------------
                  10.   SHARED DISPOSITIVE POWER

--------------------------------------------------------------------------------
11.   AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
        2,151,669*
--------------------------------------------------------------------------------
12.   CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11)         [ ]
      EXCLUDES CERTAIN SHARES

--------------------------------------------------------------------------------
13.   PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
      3.1%**
--------------------------------------------------------------------------------
14.   TYPE OF REPORTING PERSON
      PN
--------------------------------------------------------------------------------

* This amount includes shares of common stock of Hexcel Corporation (the
"Issuer") beneficially owned by Berkshire Fund V, Limited Partnership ("Fund V")
which are issuable upon the conversion of shares of series A



                                                              Page 3 of 20 pages

convertible preferred stock of the Issuer at a conversion price initially set at
$3.00 per share (each share of such preferred stock is convertible into a number
of shares of common stock equal to $1,000.00 / $3.00).

** This percentage was calculated in accordance with Rule 13-3(d)(1) of the
Exchange Act, which specifically excludes from such calculation all securities
not outstanding which are subject to options, warrants, rights or conversion
privileges and which are beneficially owned by any person other than Fund V.
Accordingly, such calculation does not include the shares of common stock of the
Issuer issuable upon conversion of shares of series A convertible stock of the
Issuer and series B convertible stock of the Issuer which are beneficially owned
by any person other than Fund V. This percentage would equal 2.4% if it were
calculated by including such securities in such calculation.

Percentage calculations are based on the number of shares outstanding upon the
closing of the Issuer's registered offering on August 9, 2005, as disclosed in
the Issuer's final prospectus filed with the Commission on August 3, 2005 (based
on the number of shares outstanding June 24, 2005), after giving effect to the
additional shares sold August 17, 2005 pursuant to the underwriters'
overallotment option in connection with such offering.



                                                              Page 4 of 20 pages

CUSIP NO. 428290 10 0

                                  SCHEDULE 13D

--------------------------------------------------------------------------------
1.    NAME OF REPORTING PERSONS
      S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSONS
      Berkshire Fund VI, Limited Partnership
      04-3568357
--------------------------------------------------------------------------------
2.    CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*

                                                                     (a)[ ]

                                                                     (b)[X]
--------------------------------------------------------------------------------
3.    SEC USE ONLY
--------------------------------------------------------------------------------
4.    SOURCE OF FUNDS
        OO
--------------------------------------------------------------------------------
5.    CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS          [ ]
      IS REQUIRED PURSUANT TO ITEM 2(d) or 2(e)

--------------------------------------------------------------------------------
6.    CITIZENSHIP OR PLACE OF ORGANIZATION

            Massachusetts
--------------------------------------------------------------------------------
                  7.    SOLE VOTING POWER
NUMBER OF                 2,337,191
SHARES            --------------------------------------------------------------
BENEFICIALLY      8.    SHARED VOTING POWER
OWNED BY
EACH              --------------------------------------------------------------
REPORTING         9.    SOLE DISPOSITIVE POWER
PERSON                    2,337,191
WITH              --------------------------------------------------------------
                  10.   SHARED DISPOSITIVE POWER

--------------------------------------------------------------------------------
11.   AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
        2,337,191*
--------------------------------------------------------------------------------
12.   CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11)         [ ]
      EXCLUDES CERTAIN SHARES

--------------------------------------------------------------------------------
13.   PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
      3.3%**
--------------------------------------------------------------------------------
14.   TYPE OF REPORTING PERSON
      PN
--------------------------------------------------------------------------------

* This amount includes shares of common stock of Hexcel Corporation (the
"Issuer") beneficially owned by Berkshire Fund VI, Limited Partnership ("Fund
VI) which are issuable upon the conversion of shares of series A convertible
preferred stock of the Issuer at a conversion price initially set at $3.00 per
share (each share of such preferred stock is convertible into a number of shares
of common stock equal to $1,000.00 / $3.00).



                                                              Page 5 of 20 pages

** This percentage was calculated in accordance with Rule 13-3(d)(1) of the
Exchange Act, which specifically excludes from such calculation all securities
not outstanding which are subject to options, warrants, rights or conversion
privileges and which are beneficially owned by any person other than Fund VI.
Accordingly, such calculation does not include the shares of common stock of the
Issuer issuable upon conversion of shares of series A convertible stock of the
Issuer and series B convertible stock of the Issuer which are beneficially owned
by any person other than Fund VI. This percentage would equal 2.6% if it were
calculated by including such securities in such calculation.

Percentage calculations are based on the number of shares outstanding upon the
closing of the Issuer's registered offering on August 9, 2005, as disclosed in
the Issuer's final prospectus filed with the Commission on August 3, 2005 (based
on the number of shares outstanding June 24, 2005), after giving effect to the
additional shares sold August 17, 2005 pursuant to the underwriters'
overallotment option in connection with such offering.



                                                              Page 6 of 20 pages

CUSIP NO. 428290 10 0

                                  SCHEDULE 13D

--------------------------------------------------------------------------------
1.    NAME OF REPORTING PERSONS
      S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSONS
      Berkshire Investors LLC
      04-3309729
--------------------------------------------------------------------------------
2.    CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*

                                                                     (a)[ ]

                                                                     (b)[X]
--------------------------------------------------------------------------------
3.    SEC USE ONLY
--------------------------------------------------------------------------------
4.    SOURCE OF FUNDS
        OO
--------------------------------------------------------------------------------
5.    CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS          [ ]
      IS REQUIRED PURSUANT TO ITEM 2(d) or 2(e)

--------------------------------------------------------------------------------
6.    CITIZENSHIP OR PLACE OF ORGANIZATION

            Massachusetts
--------------------------------------------------------------------------------
                  7.    SOLE VOTING POWER
NUMBER OF                 245,422
SHARES            --------------------------------------------------------------
BENEFICIALLY      8.    SHARED VOTING POWER
OWNED BY
EACH              --------------------------------------------------------------
REPORTING         9.    SOLE DISPOSITIVE POWER
PERSON                    245,422
WITH              --------------------------------------------------------------
                  10.   SHARED DISPOSITIVE POWER

--------------------------------------------------------------------------------
11.   AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
        245,422*
--------------------------------------------------------------------------------
12.   CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11)         [ ]
      EXCLUDES CERTAIN SHARES

--------------------------------------------------------------------------------
13.   PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
      0.4%**
--------------------------------------------------------------------------------
14.   TYPE OF REPORTING PERSON
        OO
--------------------------------------------------------------------------------

* This amount includes shares of common stock of Hexcel Corporation (the
"Issuer") beneficially owned by Berkshire Investors, LLC ("Berkshire Investors")
which are issuable upon the conversion of shares of series A



                                                              Page 7 of 20 pages

convertible preferred stock of the Issuer at a conversion price initially set at
$3.00 per share (each share of such preferred stock is convertible into a number
of shares of common stock equal to $1,000.00 / $3.00).

** This percentage was calculated in accordance with Rule 13-3(d)(1) of the
Exchange Act, which specifically excludes from such calculation all securities
not outstanding which are subject to options, warrants, rights or conversion
privileges and which are beneficially owned by any person other than Berkshire
Investors. Accordingly, such calculation does not include the shares of common
stock of the Issuer issuable upon conversion of shares of series A convertible
stock of the Issuer and series B convertible stock of the Issuer which are
beneficially owned by any person other than Berkshire Investors. This percentage
would equal 0.3% if it were calculated by including such securities in such
calculation.

Percentage calculations are based on the number of shares outstanding upon the
closing of the Issuer's registered offering on August 9, 2005, as disclosed in
the Issuer's final prospectus filed with the Commission on August 3, 2005 (based
on the number of shares outstanding June 24, 2005), after giving effect to the
additional shares sold August 17, 2005 pursuant to the underwriters'
overallotment option in connection with such offering.



                                                              Page 8 of 20 pages

CUSIP NO. 428290 10 0

                                  SCHEDULE 13D

--------------------------------------------------------------------------------
1.    NAME OF REPORTING PERSONS
      S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSONS
      Berkshire Partners LLC
      04-2911958
--------------------------------------------------------------------------------
2.    CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*

                                                                     (a)[ ]

                                                                     (b)[X]
--------------------------------------------------------------------------------
3.    SEC USE ONLY
--------------------------------------------------------------------------------
4.    SOURCE OF FUNDS
        OO
--------------------------------------------------------------------------------
5.    CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS          [ ]
      IS REQUIRED PURSUANT TO ITEM 2(d) or 2(e)

--------------------------------------------------------------------------------
6.    CITIZENSHIP OR PLACE OF ORGANIZATION

            Massachusetts
--------------------------------------------------------------------------------
                  7.    SOLE VOTING POWER
NUMBER OF                 13,031
SHARES            --------------------------------------------------------------
BENEFICIALLY      8.    SHARED VOTING POWER
OWNED BY
EACH              --------------------------------------------------------------
REPORTING         9.    SOLE DISPOSITIVE POWER
PERSON                    13,031
WITH              --------------------------------------------------------------
                  10.   SHARED DISPOSITIVE POWER

--------------------------------------------------------------------------------
11.   AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
        13,031
--------------------------------------------------------------------------------
12.   CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11)         [ ]
      EXCLUDES CERTAIN SHARES

--------------------------------------------------------------------------------
13.   PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
      0.02%*
--------------------------------------------------------------------------------
14.   TYPE OF REPORTING PERSON
      OO
--------------------------------------------------------------------------------

* This percentage was calculated in accordance with Rule 13-3(d)(1) of the
Exchange Act, which specifically excludes from such calculation all securities
not outstanding which are subject to options, warrants, rights or


                                                              Page 9 of 20 pages

conversion privileges and which are beneficially owned by any person other than
Berkshire Partners LLC ("Berkshire"). Accordingly, such calculation does not
include the shares of common stock of Hexcel Corporation (the "Issuer") issuable
upon conversion of shares of series A convertible stock of the Issuer and series
B convertible stock of the Issuer which are beneficially owned by any person
other than Berkshire. This percentage would be 0.01% if it were calculated by
including such securities in such calculation.

Percentage calculations are based on the number of shares outstanding upon the
closing of the Issuer's registered offering on August 9, 2005, as disclosed in
the Issuer's final prospectus filed with the Commission on August 3, 2005 (based
on the number of shares outstanding June 24, 2005), after giving effect to the
additional shares sold August 17, 2005 pursuant to the underwriters'
overallotment option in connection with such offering.



                                                             Page 10 of 20 pages

CUSIP NO. 428290 10 0

SCHEDULE 13D

--------------------------------------------------------------------------------
1.    NAME OF REPORTING PERSONS
      S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSONS
      Greenbriar Equity Fund, L.P.
      13-4089201
--------------------------------------------------------------------------------
2.    CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*

                                                                     (a)[ ]

                                                                     (b)[X]
--------------------------------------------------------------------------------
3.    SEC USE ONLY
--------------------------------------------------------------------------------
4.    SOURCE OF FUNDS
        OO
--------------------------------------------------------------------------------
5.    CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS          [ ]
      IS REQUIRED PURSUANT TO ITEM 2(d) or 2(e)

--------------------------------------------------------------------------------
6.    CITIZENSHIP OR PLACE OF ORGANIZATION

            Delaware
--------------------------------------------------------------------------------
                  7.    SOLE VOTING POWER
NUMBER OF                 4,641,700
SHARES            --------------------------------------------------------------
BENEFICIALLY      8.    SHARED VOTING POWER
OWNED BY
EACH              --------------------------------------------------------------
REPORTING         9.    SOLE DISPOSITIVE POWER
PERSON                    4,641,700
WITH              --------------------------------------------------------------
                  10.   SHARED DISPOSITIVE POWER

--------------------------------------------------------------------------------
11.   AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
        4,641,700*
--------------------------------------------------------------------------------
12.   CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11)         [ ]
      EXCLUDES CERTAIN SHARES

--------------------------------------------------------------------------------
13.   PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
      6.4%**
--------------------------------------------------------------------------------
14.   TYPE OF REPORTING PERSON
      PN
--------------------------------------------------------------------------------



                                                             Page 11 of 20 pages

* This amount includes shares of common stock of Hexcel Corporation (the
"Issuer") beneficially owned by Greenbriar Equity Fund, L.P. ("Greenbriar Fund")
which are issuable upon the conversion of shares of series A convertible
preferred stock of the Issuer at a conversion price initially set at $3.00 per
share (each share of such preferred stock is convertible into a number of shares
of common stock equal to $1,000.00 / $3.00).

** This percentage was calculated in accordance with Rule 13-3(d)(1) of the
Exchange Act, which specifically excludes from such calculation all securities
not outstanding which are subject to options, warrants, rights or conversion
privileges and which are beneficially owned by any person other than Greenbriar
Fund. Accordingly, such calculation does not include the shares of common stock
of the Issuer issuable upon conversion of shares of series A convertible stock
of the Issuer and series B convertible stock of the Issuer which are
beneficially owned by any person other than Greenbriar Fund. This percentage
would equal 5.1% if it were calculated by including such securities in such
calculation.

Percentage calculations are based on the number of shares outstanding upon the
closing of the Issuer's registered offering on August 9, 2005, as disclosed in
the Issuer's final prospectus filed with the Commission on August 3, 2005 (based
on the number of shares outstanding June 24, 2005), after giving effect to the
additional shares sold August 17, 2005 pursuant to the underwriters'
overallotment option in connection with such offering.



                                                             Page 12 of 20 pages

CUSIP NO. 428290 10 0

SCHEDULE 13D

--------------------------------------------------------------------------------
1.    NAME OF REPORTING PERSONS
      S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSONS
      Greenbriar Co-Investment Partners, L.P.
      13-4089202
--------------------------------------------------------------------------------
2.    CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*

                                                                     (a)[ ]

                                                                     (b)[X]
--------------------------------------------------------------------------------
3.    SEC USE ONLY
--------------------------------------------------------------------------------
4.    SOURCE OF FUNDS
        OO
--------------------------------------------------------------------------------
5.    CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS          [ ]
      IS REQUIRED PURSUANT TO ITEM 2(d) or 2(e)

--------------------------------------------------------------------------------
6.    CITIZENSHIP OR PLACE OF ORGANIZATION

            Delaware
--------------------------------------------------------------------------------
                  7.    SOLE VOTING POWER
NUMBER OF                 92,582
SHARES            --------------------------------------------------------------
BENEFICIALLY      8.    SHARED VOTING POWER
OWNED BY
EACH              --------------------------------------------------------------
REPORTING         9.    SOLE DISPOSITIVE POWER
PERSON                    92,582
WITH              --------------------------------------------------------------
                  10.   SHARED DISPOSITIVE POWER

--------------------------------------------------------------------------------
11.   AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
        92,582*
--------------------------------------------------------------------------------
12.   CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11)         [ ]
      EXCLUDES CERTAIN SHARES

--------------------------------------------------------------------------------
13.   PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
      0.1%**
--------------------------------------------------------------------------------
14.   TYPE OF REPORTING PERSON
      PN
--------------------------------------------------------------------------------



                                                             Page 13 of 20 pages

* This amount includes shares of common stock of Hexcel Corporation (the
"Issuer") beneficially owned by Greenbriar Co-Investment Partners, L.P.
("Greenbriar Co-Invest") which are issuable upon the conversion of shares of
series A convertible preferred stock of the Issuer at a conversion price
initially set at $3.00 per share (each share of such preferred stock is
convertible into a number of shares of common stock equal to $1,000.00 / $3.00).

** This percentage was calculated in accordance with Rule 13-3(d)(1) of the
Exchange Act, which specifically excludes from such calculation all securities
not outstanding which are subject to options, warrants, rights or conversion
privileges and which are beneficially owned by any person other than Greenbriar
Co-Invest. Accordingly, such calculation does not include the shares of common
stock of the Issuer issuable upon conversion of shares of series A convertible
stock of the Issuer and series B convertible stock of the Issuer which are
beneficially owned by any person other than Greenbriar Co-Invest. This
percentage would remain 0.1% if it were calculated by including such securities
in such calculation.

Percentage calculations are based on the number of shares outstanding upon the
closing of the Issuer's registered offering on August 9, 2005, as disclosed in
the Issuer's final prospectus filed with the Commission on August 3, 2005 (based
on the number of shares outstanding June 24, 2005), after giving effect to the
additional shares sold August 17, 2005 pursuant to the underwriters'
overallotment option in connection with such offering.



                                                             Page 14 of 20 pages

CUSIP NO. 428290 10 0

                                  SCHEDULE 13D

--------------------------------------------------------------------------------
1.    NAME OF REPORTING PERSONS
      S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSONS
      Greenbriar Equity Group LLC
      13-4089194
--------------------------------------------------------------------------------
2.    CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*

                                                                     (a)[ ]

                                                                     (b)[X]
--------------------------------------------------------------------------------
3.    SEC USE ONLY
--------------------------------------------------------------------------------
4.    SOURCE OF FUNDS
        OO
--------------------------------------------------------------------------------
5.    CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS          [ ]
      IS REQUIRED PURSUANT TO ITEM 2(d) or 2(e)

--------------------------------------------------------------------------------
6.    CITIZENSHIP OR PLACE OF ORGANIZATION

            Delaware
--------------------------------------------------------------------------------
                  7.    SOLE VOTING POWER
NUMBER OF                 13,031
SHARES            --------------------------------------------------------------
BENEFICIALLY      8.    SHARED VOTING POWER
OWNED BY
EACH              --------------------------------------------------------------
REPORTING         9.    SOLE DISPOSITIVE POWER
PERSON                    13,031
WITH              --------------------------------------------------------------
                  10.   SHARED DISPOSITIVE POWER

--------------------------------------------------------------------------------
11.   AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
        13,031
--------------------------------------------------------------------------------
12.   CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11)         [ ]
      EXCLUDES CERTAIN SHARES

--------------------------------------------------------------------------------
13.   PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
      0.02%*
--------------------------------------------------------------------------------
14.   TYPE OF REPORTING PERSON
      OO
--------------------------------------------------------------------------------



                                                             Page 15 of 20 pages

* This percentage was calculated in accordance with Rule 13-3(d)(1) of the
Exchange Act, which specifically excludes from such calculation all securities
not outstanding which are subject to options, warrants, rights or conversion
privileges and which are beneficially owned by any person other than Greenbriar
Equity Group LLC ("Greenbriar"). Accordingly, such calculation does not include
the shares of common stock of Hexcel Corporation (the "Issuer") issuable upon
conversion of shares of series A convertible stock of the Issuer and series B
convertible stock of the Issuer which are beneficially owned by any person other
than Greenbriar. This percentage would be 0.01% if it were calculated by
including such securities in such calculation.

Percentage calculations are based on the number of shares outstanding upon the
closing of the Issuer's registered offering on August 9, 2005, as disclosed in
the Issuer's final prospectus filed with the Commission on August 3, 2005 (based
on the number of shares outstanding June 24, 2005), after giving effect to the
additional shares sold August 17, 2005 pursuant to the underwriters'
overallotment option in connection with such offering.



                                                             Page 16 of 20 pages

      Except as set forth below, all previously reported Items are unchanged.
For clarity, Item 2(a) is restated without change in its form.

Item 2. IDENTITY AND BACKGROUND.

(a)   Name

      This Statement is being filed jointly by the following (each a "Reporting
Person" and collectively, the "Reporting Persons"): (1) Berkshire Fund V,
Limited Partnership, a Massachusetts limited partnership ("Fund V"), (2)
Berkshire Fund VI, Limited Partnership, a Massachusetts limited partnership
("Fund VI") (3), Berkshire Investors LLC, a Massachusetts limited liability
company ("Berkshire Investors"), (4) Berkshire Partners LLC, a Massachusetts
limited liability company ("Berkshire"), (5) Greenbriar Equity Fund, L.P. a
Delaware limited partnership ("Greenbriar Fund") (6) Greenbriar Co-Investment
Partners, L.P., a Delaware limited partnership ("Greenbriar Co-Invest") and (7)
Greenbriar Equity Group LLC, a Delaware limited liability company
("Greenbriar").

      Fifth Berkshire Associates LLC, a Massachusetts limited liability company
("Fifth Berkshire") is the general partner of Fund V. Sixth Berkshire Associates
LLC, a Massachusetts limited liability company ("Sixth Berkshire") is the
general partner of Fund VI. The managing members of Fifth Berkshire are: Bradley
M. Bloom, J. Christopher Clifford, Kevin T. Callaghan, Richard K. Lubin, Carl
Ferenbach, Jane Brock-Wilson, David R. Peeler, Robert J. Small, and Ross M.
Jones (the "Berkshire Principals"). The Berkshire Principals are also the
managing members of Sixth Berkshire and Berkshire Investors.

      Greenbriar Holdings LLC ("Greenbriar Holdings") is the general partner of
Greenbriar Equity Capital, L.P.("Greenbriar Capital") , which is the general
partner of Greenbriar Fund. Greenbriar Holdings is also the general partner of
Greenbriar Co-Invest. The managing members of Greenbriar Holdings are Joel S.
Beckman, Gerald Greenwald and Reginald L. Jones, III (the "Greenbriar
Principals").

      The Reporting Persons make acquisitions in, and dispose of, securities of
an issuer on the same terms and conditions and at the same time. Pursuant to the
Coinvestment Agreement (the "Coinvestment Agreement") dated as of November 1,
1999 between Berkshire and Greenbriar, Berkshire and Greenbriar agreed to pursue
jointly investment opportunities in the transportation industry. Berkshire has
agreed to cause one or more of its affiliated investment funds to co-invest with
Greenbriar Fund in an amount equal to one dollar for every two dollars invested
by Greenbriar Fund. If the amount available for investment in a particular
transaction exceeds the amount which is appropriate for Greenbriar Fund to
invest, Berkshire's affiliated funds shall be offered the opportunity to
coinvest in such transaction up to such additional amount, unless Greenbriar,
after consultation with Berkshire, determines to first offer all or a portion of
such coinvestment opportunity to one or more third parties. As a result,
Berkshire and Greenbriar agreed to invest in the transactions described in this
filing on an equal basis (the Reporting Persons affiliated with Berkshire have
invested one dollar for every one dollar invested by the Reporting Persons
affiliated with Greenbriar).

      Based on the foregoing and the transactions and relationships described
herein, the Reporting Persons may be deemed to constitute a "group" for purposes
of Section 13(d)(3) of the Securities Exchange Act of 1934 (the "Exchange Act").
The filing of this statement shall not be construed as an admission that the
Reporting Persons are a group, or have agreed to act as a group.

Item 4. PURPOSE OF TRANSACTION.

Item 4 is hereby amended by adding the following immediately before the final
paragraph thereof:

      On August 17, 2005, the Sellers sold an aggregate of 2,115,765 additional
shares of Common Stock of the Issuer in the Second Registered Offering pursuant
to the underwriters' exercise of their overallotment option. The



                                                             Page 17 of 20 pages

number of shares sold by each Seller is as follows: Fund V - 480,947, Fund VI -
522,275, Berkshire Investors - 54,660, Greenbriar Fund - 1,037,219, and
Greenbriar Co-Invest - 20,664.

      The Sellers, in the aggregate, converted 6,348 shares of series A
convertible preferred stock of the Issuer into 2,115,998 shares of Common Stock
of the Issuer, of which 2,115,765 shares were sold in the Second Registered
Offering pursuant to the underwriters' overallotment option on August 17, 2005
in connection with the Second Registered Offering.

Item 5. INTEREST IN SECURITIES OF THE ISSUER.

Item 5 is hereby amended and restated in its entirety as follows:

(a) As of the closing of the sale of shares of Common Stock sold on August 17,
2005 pursuant to the overallotment option in connection with the Second
Registered Offering (the "Second Overallotment Closing"), the Reporting Persons
may be deemed to beneficially own an aggregate of 9,494,626 shares of Common
Stock consisting of (i) 28,405 shares of series A preferred stock that are
beneficially owned by the Reporting Persons and that are convertible into
9,468,331 shares of Common Stock; (ii) 233 shares of Common Stock resulting from
the conversion of series A preferred stock for the Second Overallotment Closing
that were not sold by the underwriters in the Second Overallotment Closing;
(iii) 12,000 shares subject to currently exercisable options and 1,031 shares
issuable upon conversion of vested restricted stock units, in each case granted
to Robert J. Small pursuant to the Issuer's Incentive Stock Plan (Mr. Small has
an understanding with Berkshire pursuant to which he holds such options and
restricted stock units for the benefit of Berkshire); and (iv) 12,000 shares
subject to currently exercisable options and 1,031 shares issuable upon
conversion of vested restricted stock units, in each case granted to Joel S.
Beckman pursuant to the Issuer's Incentive Stock Plan (Mr. Beckman has an
understanding with Greenbriar pursuant to which he holds such options and
restricted stock units for the benefit of Greenbriar).

      The shares of Common Stock that may be deemed to be beneficially owned by
the Reporting Persons as of the closing of the Second Overallotment Closing
represent approximately 12.3% of the outstanding shares of Common Stock, based
on 67,887,550 shares of Common Stock outstanding, consistent with the number of
shares outstanding upon the closing of the Second Registered Offering disclosed
in the Issuer's final prospectus filed with the Commission on August 3, 2005 and
after giving effect to the shares sold at the Second Overallotment Closing and
the additional shares of preferred stock converted into Common Stock in
connection therewith. The foregoing percentage was calculated in accordance with
Rule 13-3(d)(1) of the Exchange Act, which specifically excludes from such
calculation all securities not outstanding which are subject to options,
warrants, rights or conversion privileges and which are beneficially owned by
any person other than the Reporting Persons. Accordingly, such calculation does
not include shares of series A convertible preferred stock and series B
convertible preferred stock which are beneficially owned by any person other
than the Reporting Persons. All outstanding shares of series A convertible
preferred stock and series B convertible preferred stock are entitled to vote,
on an as-converted basis, on all matters put to a vote or consent of the holders
of Common Stock. Therefore, in any such vote or consent, as of the closing of
the Second Registered Offering, the Reporting Persons have the power to vote
shares representing approximately 10.4% of the total number of votes that may be
cast on any such matter.

      By virtue of the relationships described herein, the Reporting Persons may
be deemed to constitute a "group" within the meaning of Rule 13d-5 under the
Exchange Act. As a member of a group, each Reporting Person may be deemed to
share voting and dispositive power with respect to, and therefore beneficially
own, the shares beneficially owned by members of the group as a whole. The
filing of this Statement shall not be construed as an admission that the
Reporting Persons beneficially own those shares held by any other members of the
group.

      (b) As of the Second Overallotment Closing on August 17, 2005, Fund V has
sole voting power with respect to 2,151,669 shares of Common Stock and sole
dispositive power with respect to 2,151,669 shares of Common Stock. Fund V is
the beneficial owner of the 2,151,669 shares of Common Stock over which it has
voting and dispositive power.



                                                             Page 18 of 20 pages

      As the ultimate general partner of Fund V, Fifth Berkshire may be deemed
to beneficially own 2,151,669 shares of Common Stock held by Fund V. The filing
of this statement shall not be construed as an admission that Fifth Berkshire
is, for the purpose of Section 13(d) of the Exchange Act, the beneficial owner
of such shares held by Fund V.

      As of the Second Overallotment Closing on August 17, 2005, Fund VI has
sole voting power with respect to 2,337,191 shares of Common Stock and sole
dispositive power with respect to 2,337,191 shares of Common Stock. Fund VI is
the beneficial owner of the 2,337,191 shares of Common Stock over which it has
voting and dispositive power.

      As the sole general partner of Fund VI, Sixth Berkshire may be deemed to
beneficially own 2,337,191 shares of the Common Stock held by Fund VI. The
filing of this statement shall not be construed as an admission that Sixth
Berkshire is, for the purpose of Section 13(d) of the Exchange Act, the
beneficial owner of such shares held by Fund VI.

      As of the Second Overallotment Closing on August 17, 2005, Berkshire
Investors has sole voting power with respect to 245,422 shares of Common Stock
and sole dispositive power with respect to 245,422 shares of Common Stock.
Berkshire Investors is the direct beneficial owner of the 245,422 shares of
Common Stock over which it has voting and dispositive power.

      As of the Second Overallotment Closing on August 17, 2005, Berkshire may
be deemed to beneficially own 12,000 shares subject to currently exercisable
options and 1,031 shares issuable upon conversion of vested restricted stock
units, in each case granted to Robert J. Small pursuant to the Issuer's
Incentive Stock Plan. Mr. Small holds these options and restricted stock units
for the benefit of Berkshire.

      By virtue of their positions as managing members of Fifth Berkshire, Sixth
Berkshire, Berkshire Investors and Berkshire, the Berkshire Principals may be
deemed to possess indirect beneficial ownership of the shares of Common Stock
beneficially owned by Fund V, Fund VI or Berkshire Investors. None of the
Berkshire Principals, acting alone, however, has voting or investment power with
respect to the shares beneficially owned by the Fund V, Fund VI or Berkshire
Investors, and, as a result, each Berkshire Principal disclaims beneficial
ownership of such shares of Common Stock.

      As of the Second Overallotment Closing on August 17, 2005, Greenbriar Fund
has sole voting power with respect to 4,641,700 shares of Common Stock and sole
dispositive power with respect to 4,641,700 shares of Common Stock. Greenbriar
Fund is the direct beneficial owner of the 4,641,700 shares of Common Stock over
which it has voting and dispositive power.

      As the ultimate general partner of Greenbriar Fund, Greenbriar Holdings
may be deemed to beneficially own 4,641,700 shares of the Common Stock held by
Greenbriar Fund. The filing of this statement shall not be construed as an
admission that Greenbriar Holdings is, for the purpose of Section 13(d) of the
Exchange Act, the beneficial owner of such shares held by Greenbriar Fund.

      As of the Second Overallotment Closing on August 17, 2005, Greenbriar
Co-Invest has sole voting power with respect to 92,582 shares of Common Stock
and sole dispositive power with respect to 92,582 shares of Common Stock.
Greenbriar Co-Invest is the direct beneficial owner of the 92,582 shares of
Common Stock over which it has voting and dispositive power.

      As the sole general partner of Greenbriar Co-Invest, Greenbriar Holdings
may be deemed to beneficially own 92,582 shares of the Common Stock held by
Greenbriar Co-Invest. The filing of this statement shall not be construed as an
admission that Greenbriar Holdings is, for the purpose of Section 13(d) of the
Exchange Act, the beneficial owner of such shares held by Greenbriar Co-Invest.



                                                             Page 19 of 20 pages

      As of the Second Overallotment Closing on August 17, 2005, Greenbriar may
be deemed to beneficially own 12,000 shares subject to currently exercisable
options and 1,031 shares issuable upon conversion of vested restricted stock
units, in each case granted to Joel S. Beckman pursuant to the Issuer's
Incentive Stock Plan. Mr. Beckman holds these options and restricted stock
units for the benefit of Greenbriar.

      By virtue of their positions as managing members of Greenbriar Holdings,
the Greenbriar Principals may be deemed to possess indirect beneficial ownership
of the shares of Common Stock beneficially owned by Greenbriar Fund, Greenbriar
Co-Invest or Greenbriar. None of the Greenbriar Principals, acting alone,
however, has voting or investment power with respect to the shares beneficially
owned by Greenbriar Fund or Greenbriar Co-Invest, and, as a result, each
Greenbriar Principal disclaims beneficial ownership of such shares of Common
Stock.

ITEM 7. MATERIAL TO BE FILED AS EXHIBITS.

Item 7 is hereby amended and restated in its entirety as follows:

Exhibit 1   Joint Filing Undertaking, dated as of March 28, 2003*

Exhibit 2   Stock Purchase Agreement dated as of December 18, 2002 between the
            Purchasers and the Issuer*

Exhibit 3   Stockholders Agreement dated as of March 19, 2003 between the
            Purchaser and the Issuer*

Exhibit 4   Registration Rights Agreement dated as of March 19, 2003 between the
            Purchasers and the Issuer*

Exhibit 5   Series A Certificate of Designations dated as of March 19, 2003*

Exhibit 6   Series B Certificate of Designations dated as of March 19, 2003*

Exhibit 7   Coinvestment Agreement dated November 1, 1999 between Berkshire
            Partners LLC and Greenbriar Equity Group, LLC*

Exhibit 8   Joint Filing Agreement regarding Amendment No. 1 to Schedule 13D*

Exhibit 9   Joint Filing Agreement regarding Amendment No. 2 to Schedule 13D*

Exhibit 10  Joint Filing Agreement regarding Amendment No. 3 to Schedule 13D*

Exhibit 11  Underwriting Agreement dated as of August 3, 2005 (incorporated by
            reference to Exhibit 99.1 of the Issuer's 8-K filed with the
            Commission on August 9, 2005)

Exhibit 12  Joint Filing Agreement regarding Amendment No. 4 to Schedule 13D

* previously filed



                                                             Page 20 of 20 pages

                                   SIGNATURES

            After reasonable inquiry and to the best of my knowledge and belief,
I certify that the information set forth in this Statement is true, complete and
correct.

Dated: August 18, 2005

                       BERKSHIRE FUND V, LIMITED PARTNERSHIP
                       By: Fifth Berkshire Associates LLC, its General Partner

                       BERKSHIRE FUND VI, LIMITED PARTNERSHIP
                       By: Sixth Berkshire Associates LLC, its General Partner

                       BERKSHIRE INVESTORS LLC

                       BERKSHIRE PARTNERS LLC

                       By: /s/ Robert J. Small
                           -----------------------------------
                           Name:  Robert J. Small
                           Title: Managing Director

                       GREENBRIAR EQUITY FUND, L.P.
                       By: Greenbriar Equity Capital, L.P., its general partner
                       By: Greenbriar Holdings LLC, its general partner

                       GREENBRIAR CO-INVESTMENT PARTNERS, L.P.
                       By: Greenbriar Holdings LLC, its general partner

                       GREENBRIAR EQUITY GROUP LLC

                       By: /s/ Joel S. Beckman
                           -----------------------------------
                           Name:  Joel S. Beckman
                           Title: Managing Member