Exhibit 10.6

EXECUTION COPY

WLR-GREENBRIER RAIL INC.

April 29, 2010

Greenbrier Leasing Company LLC

One Centerpointe Drive, Suite 200

Lake Oswego, Oregon 97035

To Whom It May Concern:

This letter references the Line of Credit Agreement, dated as of April 29, 2010
(the “LOC Agreement”), among WL ROSS-GREENBRIER RAIL I LLC, a Delaware limited
liability company (“Borrower”), WLR-GREENBRIER RAIL INC., a Delaware corporation
(“WLR”), AUSTRALIA AND NEW ZEALAND BANKING GROUP LIMITED, a limited liability
company incorporated under the laws of the Commonwealth of Australia (“ANZ”) and
LANDESBANK BADEN-WÜRTTEMBERG, a German public law banking institution (“LBBW”).
Each initially-capitalized term used but not defined in this letter agreement
(this “Agreement”) will have the meaning assigned to it in the LOC Agreement.

WLR agrees that:

1. It will notify Greenbrier Leasing Company LLC (“GLC”) in writing within two
Business Days after WLR receives a notice that Borrower wishes to draw an LOC
Advance under Section 2.2 of the LOC Agreement. The WLR notice to GLC will
include: (a) the total amount of the requested LOC Advance; (b) WLR’s ratable
share of the requested LOC Advance (the “Share”); (c) the LOC Drawing Date
requested by Borrower.

2. For 30 days after the LOC Drawing Date, GLC will have the right to
participate in up to [25] percent of the Share of the LOC Advance made to
Borrower on the LOC Drawing Date (“Participation Right”). GLC’s total aggregate
Participation Rights shall only apply to the Commitment of WLR under the LOC
Agreement as of the date hereof, without effect of any future amendments
increasing such amount. WLR’s initial Commitment under the LOC Agreement as of
the date hereof is $[5,250,000] and pursuant to Section 2.4(d) of the LOC
Agreement as of the date hereof may reach a maximum of $[10,500,000], such that
GLC may at most participate in up to [25]% of the $[10,500,000].

3. If GLC exercises its Participation Right within the 30-day period by written
notice to WLR (the “Election Notice”), then: (a) WLR hereby grants GLC a right
to participate in all of WLR’s rights in that percentage of the Share that is
specified by GLC (up to [25] percent of the Share) (the “Participation
Interest”).

4. The purchase price for the Participation Interest will be the dollar amount
of the Participation Interest loaned to Borrower by WLR (i.e., dollar for
dollar). WLR and GLC will close on the Participation Interest on a date agreed
to by the parties, but not later than 10

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Business Days after GLC delivers the Election Notice. From and after the date
funds are delivered to WLR pursuant to participation of the Participation
Interest, all interest, principal, fees and other amounts pertaining to such
Participation Interest shall be for the account of GLC.

5. The relationship between WLR and GLC evidenced by the Participation Interest
is and shall be that of a seller and purchaser of a participation interest in
the LOC Advance. GLC agrees that WLR will retain in WLR’s name, but to the
extent of the Participation Interest, as trustee for GLC, all of the obligations
of the Borrower to WLR arising out of the LOC Agreements. To the extent of the
Participation Interest, WLR is and shall be a trustee for GLC in administering,
servicing and taking actions under the LOC Agreement and all rights, remedies
and benefits thereunder. WLR agrees to promptly deliver to GLC copies of all
notices, statements, filings, certificates and other documents received by WLR
under the LOC Agreement. WLR agrees to pay and otherwise account for to GLC,
within five days of WLR’s receipt of any payments on any LOC Advance, such
amounts due to GLC in respect of all Participation Interests. WLR hereby agrees
that WLR shall not, without the prior written consent of GLC (with such consent
not to be unreasonably withheld) (i) carry out the provisions of the LOC
Agreement with Borrower, or exercise any rights or privileges accruing to WLR by
reason of the provisions thereof, or enforce rights thereunder; (ii) modify,
alter or amend in any material respect the LOC Agreement, nor waive or release
any material rights thereunder against Borrower, or in or to any collateral, or
in or to any material rights against any guarantor, surety or other entity in
respect of the Borrower’s obligations; or (iii) offset any claim or demand in
favor of WLR pertaining to indebtedness or obligations of Borrower wherein GLC
does not participate, against or to the detriment of GLC.

6. WLR represents and warrants that this Agreement constitutes a legal, valid
and binding obligation of WLR, enforceable against WLR in accordance with its
terms.

7. THIS AGREEMENT SHALL IN ALL RESPECTS BE GOVERNED BY, AND CONSTRUED IN
ACCORDANCE WITH, THE LAW OF THE STATE OF NEW YORK, UNITED STATES OF AMERICA,
INCLUDING ALL MATTERS OF CONSTRUCTION, VALIDITY AND PERFORMANCE WITHOUT
REFERENCE TO PRINCIPLES OF CONFLICTS OF LAW (OTHER THAN TITLE 14 OF ARTICLE 5 OF
THE GENERAL OBLIGATIONS LAW).

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If the foregoing accurately reflects our mutual understanding regarding GLC’s
Participation Right, please indicate this on the appropriate signature line
below.

 

Sincerely, WLR-GREENBRIER RAIL INC. By: /s/ Wendy Teramoto Name: Wendy Teramoto
Title: Vice President

 

Agreed and Accepted: Dated: April 29, 2010 GREENBRIER LEASING COMPANY LLC By:
/s/ James T. Sharp Name: James T. Sharp Title: President