<PAGE>

                                                                     EXHIBIT 4.6

                   Burlington Northern Santa Fe Corporation

                      8.125% Debenture due April 15, 2020

CUSIP No. 12189T AQ 7  $ 200,000,000.00

Security within the meaning of the Indenture hereinafter referred to and is
registered in the name of a Depositary or a nominee thereof. This Security may
not be exchanged in whole or in part for a Security registered, and no transfer
of this Security in whole or in part may be registered, in the name of any
Person other than such Depositary or a nominee thereof, except in the limited
circumstances described in the Indenture.

     BURLINGTON NORTHERN SANTA FE CORPORATION, a corporation duly organized and
existing under the laws of Delaware (herein called the "Company", which term
includes any successor Person under the Indenture hereinafter referred to), for
value received, hereby promises to pay to CEDE & CO. or registered assigns, the
principal sum of Two Hundred Million Dollars ($200,000,000.00) on April 15,
2020, and to pay interest thereon from April 14, 2000 or from the most recent
Interest Payment Date to which interest has been paid or duly provided for,
semi-annually on April 15 and October 15 in each year, commencing October 15,
2000, at the rate of 8.125% per annum, until the principal hereof is paid or
made available for payment. The interest so payable, and punctually paid or duly
provided for, on any Interest Payment Date will, as provided in such Indenture,
be paid to the Person in whose name this Security (or one or more Predecessor
Securities) is registered at the close of business on the Regular Record Date
for such interest, which shall be the April 1 or October 1 (whether or not a
Business Day), as the case may be, next preceding such Interest Payment Date.
Any such interest not so punctually paid or duly provided for will forthwith
cease to be payable to the Holder on such Regular Record Date and may either be
paid to the Person in whose name this Security (or one or more Predecessor
Securities) is registered at the close of business on a Special Record Date for
the payment of such Defaulted Interest to be fixed by the Trustee, notice
whereof shall be given to Holders of Securities of this series not less than 10
days prior to such Special Record Date, or be paid at any time in any other
lawful manner not inconsistent with the requirements of any securities exchange
on which the Securities of this series may be listed, and upon such notice as
may be required by such exchange, all as more fully provided in said Indenture.

     Payment of the principal of (and premium, if any) and interest on this
Security will be made at the office or agency of the Company maintained for that
purpose in the Borough of Manhattan, The City of New York, in such coin or
currency of the United States of America as at the time of payment is legal
tender for payment of public and private debts; provided, however, that at the
option of the Company payment of interest may be made by check mailed to the
address of the Person entitled thereto as such address shall appear in the
Security Register.

     Reference is hereby made to the further provisions of this Security set
forth on the reverse hereof, which further provisions shall for all purposes
have the same effect as if set forth at this place.

     Unless the certificate of authentication hereon has been executed by the
Trustee referred to on the reverse hereof by manual signature, this Security
shall not be entitled to any benefit under the Indenture or be valid or
obligatory for any purpose.

     In Witness Whereof, the Company has caused this instrument to be duly
executed under its corporate seal.

Dated:    April 14, 2000                BURLINGTON NORTHERN SANTA FE CORPORATION

  By.................................
  Thomas N. Hund
                                          Senior Vice President, Chief Financial
Officer and Treasurer
Attest:

 .........................
    Jeffrey T. Williams
    Assistant Secretary
<PAGE>

     This is one of the Securities of the series designated therein referred to
in the within-mentioned Indenture.

                                             BANK ONE TRUST COMPANY,
                                             NATIONAL ASSOCIATION
                                                                      As Trustee

  By..........................


     Authorized Officerthe Company (herein called the "Securities"), issued and
to be issued in one or more series under an Indenture, dated as of December 1,
1995 (herein called the "Indenture", which term shall have the meaning assigned
to it is such instrument), between the Company and Bank One Trust Company,
National Association, as successor to The Fir sNationalBank of Chicago, as
Trustee (herein called the "Trustee", which term includes any successor trustee
under the Indenture), and
reference is hereby made to the Indenture for a statement of the respective
rights, limitations of rights, duties and immunities thereunder of the Company,
the Trustee and the Holders of the Securities and of the terms upon which the
Securities are, andare to be, authenticated and delivered. This Security is one
of the series designated on the face hereof, limited in aggregat principal
amount to $200,000,000.00

     The Securities of this series are subject to redemption upon not less than
30 and not more than 60 days' notice by mail, at any time, as a whole or in
part, at the election of the Company, at a redemption price equal to the greater
of (i) 100% of their principal amount or (ii) the sum of the present values of
the remaining scheduled payments of principal and interest thereon discounted to
the date of redemption on a semiannual basis (assuming a 360-day year consisting
of twelve 30-day months) at the Treasury Rate (as defined in the Officers'
Certificate establishing the Securities of this series), plus 25 basis points,
plus in either case accrued interest to the date of redemption.

     In the event of redemption of this Security in part only, a new Security or
Securities of this series and of like tenor for the unredeemed portion hereof
will be issued in the name of the Holder, upon the cancellation hereof.

     The Indenture contains provisions for defeasance at any time of the entire
indebtedness of this Security or certain restrictive covenants and Events of
Default with respect to this Security, in each case upon compliance with certain
conditions set forth in the Indenture.

     If an Event of Default with respect to Securities of this series shall
occur and be continuing, the principal of the Securities of this series may be
declared due and payable in the manner and with the effect provided in the
Indenture.

     The Indenture permits, with certain exceptions as therein provided, the
amendment thereof and the modification of the rights and obligations of the
Company and the rights of the Holders of the Securities of each series to be
affected under the Indenture at any time by the Company and the Trustee with the
consent of the Holders of a majority in principal amount of the Securities at
the time Outstanding of each series to be affected. The Indenture also contains
provisions permitting the Holders of specified percentages in principal amount
of the Securities of each series at the time Outstanding, on behalf of the
Holders of all Securities of such series, to waive compliance by the Company
with certain provisions of the Indenture and certain past defaults under the
Indenture and their consequences. Any such consent or waiver by the Holder of
this Security shall be conclusive and binding upon such Holder and upon all
future Holders of this Security and of any Security issued upon the registration
of transfer hereof or in exchange herefor or in lieu hereof, whether or not
notation of such consent or waiver is made upon this Security.

     As provided in and subject to the provisions of the Indenture, the Holder
of this Security shall not have the right to institute any proceeding with
respect to the Indenture or for the appointment of a receiver or trustee or for
any other remedy thereunder, unless such Holder shall have previously given the
Trustee written notice of a continuing Event of Default with respect to the
Securities of this series, the Holders of not less than 25% in principal amount
of the Securities of this series at the time Outstanding shall have made written
request to the Trustee to institute proceedings in respect of such Event of
Default as Trustee and offered the Trustee reasonable indemnity, and the Trustee
shall not have received from the Holders of a majority in principal amount of
Securities of this series at the time Outstanding a direction inconsistent with
such request, and the Trustee shall have failed to institute any such
proceeding, for 60 days after receipt of such notice, request and offer of
indemnity. The foregoing shall not apply to any suit instituted by the Holder of
this Security for the enforcement of any payment of principal hereof or any
premium or interest hereon on or after the respective due dates expressed
herein.
<PAGE>

no provision of this Security or of the Indenture shall alter or impair the
obligation of the Company, which is absolute and unconditional, to pay the
principal of and any premium and interest on this Security at the times, place
and rate, and in the coin or currency, herein prescribed.

     As provided in the Indenture and subject to certain limitations therein set
forth, the transfer of this Security is registrable in the Security Register,
upon surrender of this Security for registration of transfer at the office or
agency of the Company in any place where the principal of and any premium and
interest on this Security are payable, duly endorsed by, or accompanied by a
written instrument of transfer in form satisfactory to the Company and the
Security Registrar duly executed by, the Holder hereof or his attorney duly
authorized in writing, and thereupon one or more new Securities of this series
and of like tenor, of authorized denominations and for the same aggregate
principal amount, will be issued to the designated transferee or transferees.

     The Securities of this series are issuable only in registered form without
coupons in denominations of $1,000 and any integral multiple thereof. As
provided in the Indenture and subject to certain limitations therein set forth,
Securities of this series are exchangeable for a like aggregate principal amount
of Securities of this series and of like tenor of a different authorized
denomination, as requested by the Holder surrendering the same.

     No service charge shall be made for any such registration of transfer or
exchange, but the Company may require payment of a sum sufficient to cover any
tax or other governmental charge payable in connection therewith.

     Prior to due presentment of this Security for registration of transfer, the
Company, the Trustee and any agent of the Company or the Trustee may treat the
Person in whose name this Security is registered as the owner hereof for all
purposes, whether or not this Security is overdue, and neither the Company, the
Trustee nor any such agent shall be affected by notice to the contrary.

     All terms used in this Security which are defined in the Indenture shall
have the meanings assigned to them in the Indenture.