<PAGE>
 
                                                                   Exhibit 4.2.2

                                                                  Conformed Copy




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                                  XTRA, INC.,

                                                   Issuer

                               XTRA CORPORATION,

                                                   Guarantor

                                      TO

                     STATE STREET BANK AND TRUST COMPANY,

                                                   Trustee

                           ------------------------

                         SECOND SUPPLEMENTAL INDENTURE

                           Dated as of May 16, 1997

                           -------------------------

                         Supplemental to the Indenture

                          Dated as of August 15, 1994

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                        SEE USE OF DEFINED TERMS BELOW

          SECOND SUPPLEMENTAL INDENTURE, dated as of May 16, 1997, among XTRA,
INC., a corporation duly organized and validly existing under the laws of the
State of Maine (herein called the "Company"), having its principal executive
office at c/o X-L-Co., Inc., 60 State Street, Boston, Massachusetts, XTRA
CORPORATION, a corporation duly organized and validly existing under the laws of
the state of Delaware (herein called "XTRA" or the "Guarantor"), having its
principal executive offices at c/o X-L-Co., Inc., 60 State Street, Boston,
Massachusetts, and STATE STREET BANK AND TRUST COMPANY, a trust company
organized and existing under the laws of the Commonwealth of Massachusetts
(herein called the "Trustee"), having its corporate trust office at Two
International Place, Boston, Massachusetts.

                                   RECITALS

     WHEREAS, the Company and the Guarantor have entered into an Indenture dated
as of August 15, 1994 (the "Original Indenture") with the Trustee (as successor
in interest to the First National Bank of Boston) to provide for the issuance
from time to time of the Company's unsecured debentures, notes or other
evidences of indebtedness (herein called the "Securities"), to be issued in one
or more series and to provide for the guarantee of the Securities by the
Guarantor (the "Guarantee");

     WHEREAS, in September 1995 the Guarantor transferred its properties and
assets substantially as an entirety to XTRA Missouri, Inc. ("XTRA Missouri"), a
wholly-owned subsidiary of the Guarantor;

     WHEREAS, the Company, as a result of such transfer, became a wholly-owned
subsidiary of XTRA Missouri, and, accordingly, remained an indirect wholly-owned
subsidiary of the Guarantor;

     WHEREAS, the Company, the Guarantor, XTRA Missouri, and the Trustee (as
successor in interest to the First National Bank of Boston) entered into the
First Supplemental Indenture dated as of September 30, 1995 (the "First
Supplemental Indenture" and together with the original Indenture, the
"Indenture"), to provide for the Guarantee of the Securities by XTRA Missouri;
 
     WHEREAS, XTRA Missouri has been merged with and into the Guarantor
effective September 30, 1996 (the "Merger"); and

     WHEREAS, there has been filed with the Trustee:  (a)  an Opinion of Counsel
in accordance with Sections 8.1 and 9.3 of the Indenture; and (b) an Officer's
Certificate in accordance with the provisions of Sections 8.1 and 1.2  of the
Indenture and the parties hereto wish to enter into this Second Supplemental
Indenture to take into consideration the Merger.

     NOW, THEREFORE, in consideration of the premises and for other good and
valuable consideration, the receipt and sufficiency of which are hereby
acknowledged, the parties hereby agree, covenant, represent and warrant as
follows:

     Section 1.  Definitions in Indenture.  Capitalized terms used in this
                 ------------------------                                 
Second Supplemental Indenture and in any certificate or other document executed
by any party in connection herewith shall have the meaning set forth in the
Original Indenture, unless a different meaning is set forth herein, in which
case such terms shall have the meaning set forth herein.

     Section 2.  Continuance of Obligations and Covenants by the Guarantor.  The
                 ---------------------------------------------------------      
Guarantor, by its execution hereof, hereby expressly affirms that it shall
remain a Guarantor of the Securities and that it shall punctually perform and
observe every obligation and covenant of the Guarantees and Indenture on the
part of the Guarantor to be performed or observed.

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     Section 3.  Modification of Forms of Series C Medium-Term Notes.  The forms
                 ---------------------------------------------------            
of the Series C Medium-Term Notes (the "Series C Notes") attached to the
resolutions of the Note Committee of the Company effective August 11, 1994 and
the resolutions of the Note Guaranty Committee of XTRA effective August 11, 1994
as exhibits, as amended by the First Supplemental Indenture, are hereby amended
to read in their respective entirety as set forth in the forms of fixed-rate and
floating rate Series C Notes attached as Exhibits 1-A and 1-B to this Second
Supplemental Indenture.  The terms of which Exhibits 1-A and 1-B are hereby
incorporated by reference and are made a part of this Second Supplemental
Indenture.

     Section 4.  Delivery of Notes.  Upon execution and delivery of this Second
                 -----------------                                             
Supplemental Indenture, the Company shall execute and deliver Series C Notes to
the Trustee and the Trustee shall authenticate the Series C Notes and deliver
them to the Holders of the Outstanding Series C Notes upon the direction of the
Company and upon receipt of the Outstanding Series C Notes, which shall
thereupon be canceled and destroyed by the Trustee. Interest on each Outstanding
Series C Note shall accrue from the last Interest Payment Date upon which
interest shall have been paid or duly provided for on the Outstanding Series C
Notes.  Prior to delivery by the Trustee of said Notes there shall be filed with
the Trustee:  (a) an Opinion of Counsel in accordance with Sections 8.1 and 9.3
of the Original Indenture and (b) an Officers' Certificate in accordance with
the provisions of Sections 1.2 and 8.1 of the Original Indenture.

     Section 5.  Recitals.  The recitals contained in this Second Supplemental
                 --------                                                     
Indenture shall be taken as statements of the Company and the Guarantor, and the
Trustee assumes no responsibility for their correctness.  The Trustee makes no
representations as to the validity or sufficiency of this Second Supplemental
Indenture.

     Section 6.  Incorporation of Indenture.  From and after the date hereof the
                 --------------------------                                     
Original Indenture, as supplemented and amended by the First Supplemental
Indenture and by this Second Supplemental Indenture, shall be read, taken and
construed as one and the same instrument with respect to the Securities.

     Section 7.  Counterparts.  This Second Supplemental Indenture may be
                 ------------                                            
executed in any number of counterparts, each of which when so executed shall be
deemed to be an original but all such counterparts together constitute but one
in the same instrument.

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     IN WITNESS WHEREOF, the parties hereto have caused this Second Supplemental
Indenture to be duly executed, and the respective seals to be hereunto affixed
and attested all as of the date(s) set forth below.

                                         XTRA, INC.

 
 
Attest:    /s/ James R. Lajoie        By:  /s/ Michael J. Soja
           -------------------             ----------------------
Title:     Assistant Clerk                  Vice President and Chief 
Date:      May 12, 1997                     Financial Officer        

                                            XTRA CORPORATION



Attest:    /s/ James R. Lajoie        By:  /s/ Michael J. Soja
           -------------------             -------------------
Title:     Assistant Clerk                 Vice President and Chief Financial
Date:      May 12, 1997                    Officer


                                           STATE STREET BANK AND TRUST COMPANY,
                                           as Trustee

 
 
Attest:    /s/ Eric  Donaghey              By:  /s/ Henry W. Seemore
           ------------------------             --------------------
Title:     Assistant Vice President           Title:  Assistant Vice President
Date:      May 13, 1997

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County of Suffolk              )
                                    ss:
Commonwealth of Massachusetts  )

     On the 12th day of  May, 1997, before me personally came Michael J. Soja,
to me known, who, being by me duly sworn, did depose and say that he is the Vice
President, Finance and Chief Financial Officer of XTRA CORPORATION, one of the
corporations described in and which executed the foregoing instrument; that he
knows the seal of said corporation; that the seal affixed to said instrument is
such corporate seal; that it was so affixed by authority of the Board of
Directors of said corporation, and that he signed his name thereto by like
authority.


 
                                    /s/ Thomas A. Giacchetto
                                    ------------------------
                                    Notary Public
                                    My commission expires:  November 10, 2000

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County of Suffolk              )
                                    ss:
Commonwealth of Massachusetts  )

     On the 12th day of May, 1997, before me personally came Michael J. Soja, to
me known, who, being by me duly sworn, did depose and say that he is the Vice
President and Chief Financial Officer of XTRA, INC., one of the corporations
described in and which executed the foregoing instrument; that he knows the seal
of said corporation; that the seal affixed to said instrument is such corporate
seal; that it was so affixed by authority of the Board of Directors of said
corporation, and that he signed his name thereto by like authority.


 
                                    /s/ Thomas A. Giacchetto
                                    ------------------------
                                    Notary Public
                                    My commission expires:  November 10, 2000


County of Suffolk              )
                                    ss:
Commonwealth of Massachusetts  )

     On the 13th day of May, 1997, before me personally came Henry W. Seemore,
to me known, who, being by me duly sworn, did depose and say that he is,
Assistant Vice President of STATE STREET BANK AND TRUST COMPANY, one of the
corporations described in and which executed the foregoing instrument; that he
knows the seal of said corporation; that the seal affixed to said instrument is
such corporate seal; that it was so affixed by authority of the Board of
Directors of said corporation, and that he signed his name thereto by like
authority.


 
                                    /s/ Stacye M. Junior
                                    --------------------
                                    Notary Public
                                    My commission expires:  September 13, 2002

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