<PAGE>   1
 
                                                                   EXHIBIT 10.19
 
                                PROMISSORY NOTE
 
$37,000,000.00                                         Dated: September 30, 1998
 
     FOR VALUE RECEIVED, the undersigned, LAKEHEAD PIPE LINE COMPANY, LIMITED
PARTNERSHIP, a Delaware limited partnership (the "Borrower"), HEREBY PROMISES TO
PAY to the order of LAKEHEAD PIPE LINE COMPANY, INC., a Delaware corporation
(the "Lender") at its offices in Duluth, Minnesota or as otherwise directed, in
lawful money of the United States of America and in immediately available funds,
the principal sum of THIRTY SEVEN MILLION AND NO/100 DOLLARS ($37,000,000.00),
together with interest on the outstanding principal balance from day to day
remaining unpaid as herein specified in monthly installments as follows:
 
          (a) On October 1, 1998, and continuing monthly and regularly
     thereafter on the first day of each and every month until September 1,
     2013, interest only at the Loan Interest Rate on the outstanding principal,
     with the interest rate being adjusted on each Interest Rate Adjustment
     Date, shall be due and payable; and
 
          (b) A final installment in the amount of all outstanding principal,
     plus accrued and unpaid interest, shall be due and payable on the Maturity
     Date.
 
                                    INTEREST
 
     Each change in the rate of interest charged hereunder shall become
effective, without notice to Borrower, on each Interest Rate Adjustment Date.
Without limiting the foregoing, Lender shall use its best efforts to notify
Borrower of any change in the interest rate hereunder, and until Borrower is
notified by Lender of such change, Borrower may continue to make payments
hereunder at the prior interest rate; provided, however, nothing contained
herein shall be construed as limiting Lender's right to collect interest at the
rates described in this Note and Borrower will promptly pay any shortfall
occasioned by Borrower's underpayment of interest on demand. In the event of
overpayment, Lender will apply the overpayment against the next accruing payment
of interest.
 
     It is the intention of the parties hereto that the Lender shall conform
strictly to usury laws applicable to it. Accordingly, if the transactions
contemplated hereby would be usurious as to the Lender under laws applicable to
it (including the laws of the United States of America or any other jurisdiction
whose laws may be mandatorily applicable to the Lender notwithstanding the other
provisions of this Note) then, in that event, notwithstanding anything to the
contrary herein, it is agreed as follows:
 
          (a) the aggregate of all consideration which constitutes interest
     under applicable law that is contracted for, taken, reserved, charged or
     received by the Lender shall under no circumstances exceed the maximum
     amount allowed by such applicable law, and any excess shall be canceled
     automatically, and if theretofore paid, shall be credited by the Lender on
     the principal amount outstanding hereunder (or, to the extent that the
     principal amount outstanding hereunder shall have been or would thereby be
     paid in full, refunded by Lender to Borrower); and
 
          (b) in the event that the Maturity Date is accelerated by reason of
     any Event of Default, or in the event of any prepayment, then such
     consideration that constitutes interest under applicable law may never
     include more than the maximum amount allowed by such applicable law, and
     excess interest, if any, provided for herein shall be canceled
     automatically by the Lender as of the date of such acceleration or
     prepayment and, if theretofore paid, shall be credited by the Lender on the
     principal amount outstanding hereunder (or, to the extent that the
     principal amount shall have been or would thereby be paid in full, refunded
     by Lender to the Borrower).
 
     All sums paid or agreed to be paid to the Lender for the use, forbearance
or detention of sums due hereunder shall, to the extent permitted by applicable
law, be amortized, prorated, allocated and spread in
<PAGE>   2
 
equal parts throughout the full term hereof until payment in full so that the
rate or amount of interest on account of the principal outstanding hereunder
does not exceed the maximum amount and rate of interest allowed by such
applicable law. If at any time and from time to time (i) the amount of interest
payable to the Lender on any date shall be computed at the Highest Lawful Rate,
and (ii) in respect of any subsequent interest computation period the amount of
interest otherwise payable to the Lender would be less than the amount of
interest payable to the Lender computed at the Highest Lawful Rate, then the
amount of interest payable to the Lender in respect of such subsequent interest
computation period shall continue to be computed at the Highest Lawful Rate
until the total amount of interest payable hereunder shall equal the total
amount of interest which would have been payable if the total amount of interest
had been computed without giving effect to this paragraph.
 
                                  DEFINITIONS
 
     As used in this Note, the following terms shall have the respective
meanings indicated below:
 
     "Acquired Assets" shall mean, at any date of determination, any assets
other than Newly Constructed Assets acquired by the Borrower or any of its
Subsidiaries from any Person at any time during the four consecutive calendar
quarter period used to determine Consolidated Cash Flow as at such date of
determination.
 
     "Applicable Margin" means the following percentages for the specified
levels, which shall be determined on each Quarterly Reporting Date for the
following calendar quarter:
 
                                 PRICING CHART
 
<TABLE>
<CAPTION>
                           LEVELS                               APPLICABLE MARGIN
                           ------                               -----------------
<S>                                                             <C>
I...........................................................           .50%
II..........................................................          .625%
III.........................................................         .8125%
IV..........................................................          1.25%
V...........................................................          2.50%
</TABLE>
 
     For purposes of the foregoing Pricing Chart, the applicable levels shall be
     determined as set forth below. The Cash Flow/Interest Coverage Pricing
     Ratio will be determined by the stated ratio on each Quarterly Certificate.
     In the event that Borrower does not deliver a Quarterly Certificate on any
     Quarterly Reporting Date, Level V shall apply during the following quarter
     until such Quarterly Certificate is delivered.
 
          Level I shall be applicable, as at any date of determination, if the
     Cash Flow/Interest Coverage Pricing Ratio as at such date shall be equal to
     or greater than 3.75;
 
          Level II shall be applicable, as at any date of determination, if the
     Cash Flow/Interest Coverage Pricing Ratio as at such date shall be equal to
     or greater than 3.00 but less than 3.75;
 
          Level III shall be applicable, as at any date of determination, if the
     Cash Flow/Interest Coverage Pricing Ratio as at such date shall be equal to
     or greater than 2.50 but less than 3.00;
 
          Level IV shall be applicable, as at any date of determination, if the
     Cash Flow/Interest Coverage Pricing Ratio as at such date shall be equal to
     or greater than 2.25 but less than 2.50; and
 
          Level V shall be applicable, as at any date of determination, if the
     Cash Flow/Interest Coverage Pricing Ratio as at such date shall be less
     than 2.25.
 
     "Business Day" shall mean any day excluding Saturday, Sunday, and any other
day on which banks are required or authorized to close in New York City, New
York or Chicago, Illinois and, if the applicable day relates to LIBOR Rate
Loans, on which trading is carried on by and between banks in Dollar deposits in
the interbank eurodollar market.
<PAGE>   3
 
     "Capital Lease" shall mean, as applied to any Person, any lease of any
property (whether real, personal or mixed) by such Person (as lessee or
guarantor or other surety) which would, in accordance with GAAP, be required to
be classified and accounted for as a capital lease on a balance sheet of such
Person.
 
     "Cash Flow/Interest Coverage Pricing Ratio" means, as of the end of each
calendar quarter, the quotient obtained by dividing (A) the sum of (a) EBITDA
plus (b) to the extent not included in EBITDA, EBITDA with respect to Acquired
Assets, excluding, however, (c) any nonrecurring amounts (including, without
limitation, amounts paid to shippers (or accrued) pursuant to decisions of
regulatory bodies or negotiated settlement agreements) to the extent included in
the foregoing clauses (a) and (b), less (d) the amount of investment income
generated by such Person and its Subsidiaries on a consolidated basis from the
investment of cash and cash equivalents, in each case for the period of four
consecutive calendar quarters ending on such calendar quarter ending date by (B)
the sum of (x) the Consolidated Gross Interest Expense less (y) the amount of
investment income generated by such Person and its Subsidiaries on a
consolidated basis from the investment of cash and cash equivalents for the same
four consecutive calendar quarter period.
 
     "Consolidated Cash Flow" shall mean, at any date of determination, (a) all
cash receipts of the Borrower and its Subsidiaries from operations (except in
respect of Acquired Assets or Newly Constructed Assets which are treated below
in this definition) during the period of four consecutive calendar quarters most
recently ended prior to such date of determination, but excluding (A) cash
proceeds from Interim Capital Transactions and (B) net cash receipts from
operations in respect of assets sold pursuant to Section 8.07(c), less (b) the
amount of investment income received by such Person and its Subsidiaries on a
consolidated basis during such period from the investment of cash and cash
equivalents, less (c) the sum of:
 
          (i) all cash operating expenditures of the Borrower and its
     Subsidiaries during such period (including, without limitation, cash
     operating expenditures of Subsidiaries prior to the acquisition thereof by
     the Borrower and taxes paid by the Borrower as an entity and by its
     Subsidiaries during such period),
 
          (ii) an amount equal to the actual reserves, if any, established by
     the general partner of the Borrower during such period for the incremental
     revenues collected by the Borrower and its Subsidiaries during such period
     pursuant to a rate increase which revenues are, at such date of
     determination, subject to possible refund,
 
          (iii) the amount, if any, by which cash reserves outstanding as of the
     end of the period that the general partner of the Borrower determines in
     its reasonable discretion to be necessary or appropriate to provide for the
     future cash payment of expenditures of the type referred to in clause (i)
     above exceeds such cash reserves outstanding at the beginning of such
     period, plus
 
          (iv) the amount, if any, by which cash reserves outstanding at the end
     of the period that the general partner of the Borrower determines in its
     reasonable discretion to be necessary or appropriate to provide funds for
     distributions with respect to the four calendar quarters following the end
     of such period exceeds such cash reserves outstanding at the beginning of
     such period,
 
plus (d) with respect to Acquired Assets, an amount equal to the cash receipts
generated by such Acquired Assets (less actual cash operating expenditures paid
with respect to such Acquired Assets) during the four consecutive calendar
quarters ending on the date of determination (regardless of the ownership
thereof during such period), plus (e) with respect to Newly Constructed Assets,
an amount equal to the product obtained by multiplying (i) the cost thereof by
(ii) the interest rate applicable to United States Treasury Bonds with a
maturity of 30 years (which interest rate shall be determined as of the
applicable date of determination) plus 1%, all as determined on a consolidated
basis and after elimination of intercompany items.
 
     "Consolidated Gross Interest Expense" means, as of the end of any calendar
quarter, Consolidated Interest Expense plus the amount of capitalized interest
(as determined in accordance with GAAP) of the Borrower and its consolidated
Subsidiaries during the same four calendar quarters used to determine
Consolidated Interest Expense as of such calendar quarter-end.
 
     "Consolidated Interest Expense" means, as of the end of any calendar
quarter, the interest expense of the Borrower and its consolidated Subsidiaries
incurred for the financing of the ordinary course of business
<PAGE>   4
 
operations of the Borrower and its consolidated Subsidiaries during the period
of the four most recently completed calendar quarters ending on such calendar
quarter ending date (as determined in accordance with GAAP).
 
     "EBITDA" shall mean, for any period, (a) Net Income for such period plus
(b) depreciation, amortization, interest expense, and income taxes for such
period, in each case to the extent deducted in determining Net Income for such
period, all as determined on a consolidated basis for the Borrower and its
Subsidiaries.
 
     "GAAP" shall mean generally accepted accounting principles in effect in the
United States from time to time.
 
     "Highest Lawful Rate" means the maximum nonusurious interest rate that may
under applicable law be contracted for, charged, received, taken or reserved by
Lender in connection with loans made by Lender.
 
     "Indebtedness" of any Person shall mean (without duplication):
 
          (a) any indebtedness for borrowed money which such Person has directly
     or indirectly created, incurred or assumed;
 
          (b) any indebtedness, whether or not for borrowed money, secured by
     any Lien in respect of property owned by such Person, whether or not such
     Person has assumed or become liable for the payment of such indebtedness,
     provided that the amount of such Indebtedness if not so assumed shall in no
     event be deemed to be greater than the fair market value from time to time
     (as determined in good faith by such Person) of the property subject to
     such Lien;
 
          (c) any indebtedness, whether or not for borrowed money, with respect
     to which such Person has become directly or indirectly liable and which
     represents the deferred purchase price (or a portion thereof) or has been
     incurred to finance the purchase price (or a portion thereof) of any
     property, service or business acquired by such Person, whether by purchase,
     consolidation, merger or otherwise, excluding, however, trade accounts
     payable incurred in the ordinary course of business by the Person whose
     Indebtedness is being determined;
 
          (d) any obligations under Capital Leases to the extent such
     obligations would, in accordance with GAAP, appear on a balance sheet of
     such Person;
 
          (e) any indebtedness of the character referred to in clause (a), (b),
     or (d) of this definition deemed to be extinguished under GAAP but for
     which such Person remains legally liable; and
 
          (f) any indebtedness of any other Person of the character referred to
     in clause (a), (b), (c), (d) or (e) of this definition with respect to
     which the Person whose Indebtedness is being determined has become liable
     by way of a Guaranty.
 
     "Interest Rate Adjustment Date" means the date hereof and the first day of
each and every month thereafter prior to the Maturity Date.
 
     "Interim Capital Transactions" shall mean (a) borrowings and sales of debt
securities (other than for working capital purposes and other than for items
purchased on open account in the ordinary course of business) by the Borrower,
(b) sales of equity interests by the Borrower or capital contributions to the
Borrower, and (c) sales or other voluntary or involuntary dispositions of any
assets of the Borrower (other than (i) sales or other dispositions of inventory
in the ordinary course of business, (ii) sales or other dispositions of other
current assets including accounts receivable or (iii) sales or other
dispositions of assets as a part of normal retirements or replacements), in each
case prior to the commencement of the dissolution and liquidation of the
Borrower.
 
     "Lien" shall mean, as to any Person, any mortgage, lien (statutory or
otherwise), pledge, reservation, right of entry, encroachment, easement,
right-of-way, restrictive covenant, license, charge, security interest or other
encumbrance in or on, or any interest or title of any vendor, lessor, lender or
other secured party to or of such Person under any conditional sale or other
title retention agreement or Capital Lease with respect to, any
<PAGE>   5
 
property or asset owned or held by such Person, or the signing or filing of a
financing statement with respect to any of the foregoing which names such Person
as debtor, or the signing of any security agreement with respect to any of the
foregoing authorizing any other party as the secured party thereunder to file
any financing statement or any other agreement to give or grant any of the
foregoing. For the purposes of this Note, a Person shall be deemed to be the
owner of any asset which it has placed in trust for the benefit of the holders
of Indebtedness of such Person and such trust shall be deemed to be a Lien if
such Indebtedness is deemed to be extinguished under GAAP and such Person
remains legally liable therefor.
 
     "Loan Interest Rate" means from the date hereof until the next Interest
Rate Adjustment Date 8.75% per annum and thereafter means the lesser of (i) the
Highest Lawful Rate, or (ii) the sum of the LIBOR Rate plus the Applicable
Margin.
 
     "LIBOR Rate" means the rate per annum (rounded to 1/16 of 1%) at which
dollar deposits approximately equal in principal amount to the entire principal
amount of this Note and for a period of one (1) month are offered, as quoted on
the display designated as page 4136 on the Telerate Service (or such other
display as may replace Page 4136 on the Telerate Service), or such other
nationally-recognized rate quoting service selected by Lender.
 
     "Maturity Date" means September 30, 2013.
 
     "Net Income" shall mean, for any period, the net earnings income (or loss)
after taxes for such period taken as a single accounting period on a
consolidated basis for the Borrower and its Subsidiaries determined in
accordance with GAAP.
 
     "Newly Constructed Assets" shall mean, at any date of determination, any
pipeline assets and facilities related thereto which are intended to be included
in the common carrier rate base of the Borrower and which are then being either
constructed by or on behalf of the Borrower or any of its Subsidiaries or if
already owned by the Borrower or its Subsidiaries, substantially rehabilitated
or enhanced.
 
     "Person" means any individual, partnership, firm, corporation, association,
joint venture, trust or other entity or enterprise, or any governmental or
political subdivision or agency, department or instrumentality thereof.
 
     "Quarterly Certificate" means a certificate signed by an officer of the
general partner of Borrower, stating (i) what the Cash Flow/Interest Coverage
Pricing Ratio is for the immediately preceding calendar quarter and (ii) that
there exists no Event of Default hereunder.
 
     "Quarterly Reporting Date" means the first day of each January, April, July
and November during the term hereof.
 
     "Subsidiary" shall mean any corporation, association, partnership, joint
venture or other business entity at least a majority (by number of votes) of the
stock of any class or classes (or equivalent interests) of which is at the time
owned by a Person or by one or more Subsidiaries of such Person or by a Person
and one or more Subsidiaries of such Person, if the holders of the stock of such
class or classes (or equivalent interests) (a) are ordinarily, in the absence of
contingencies, entitled to vote for the election of a majority of the directors
(or Persons performing similar functions) of such business entity, even though
the right so to vote has been suspended by the happening of such a contingency,
or (b) are at the time entitled, as such holders, to vote for the election of
the majority of the directors (or Persons performing similar functions) of such
business entity, whether or not the right so to vote exists by reason of the
happening of a contingency.
 
                                   COVENANTS
 
     During the term hereof, Borrower shall comply with the following covenants:
 
          (a) Borrower shall deliver to Lender a Quarterly Certificate on each
     Quarterly Reporting Date.
 
          (b) Borrower will at all times preserve and keep in full force and
     effect its partnership existence.
<PAGE>   6
 
          (c) Borrower will at all times comply with all laws and regulations
     applicable to it, the failure with which to comply, individually or in the
     aggregate, would materially adversely affect its business or its
     operations.
 
          (d) Borrower will, and will cause each Subsidiary to, pay all taxes,
     assessments and other governmental charges imposed upon it or any of its
     properties or assets or in respect of any of its franchises, business,
     income or profits when the same become due and payable, but in any event
     before any penalty or interest accrues thereon, and all claims (including,
     without limitation, claims for labor, materials and supplies) for sums
     which have become due and payable and which by law have or might become a
     Lien upon any of its properties or assets; provided that no such tax,
     assessment, charge or claim need be paid or reimbursed if being contested
     in good faith by appropriate proceedings properly initiated and diligently
     conducted and if such reserves or other appropriate provision, if any, as
     shall be required by GAAP shall have been made therefor and be adequate in
     its good faith judgment.
 
          (e) Borrower will maintain or cause to be maintained in good repair,
     working order and condition all properties used or useful in its business
     and that of its Subsidiaries and from time to time will make or cause to be
     made all appropriate repairs, renewals and replacements thereof. Borrower
     will maintain or cause to be maintained, with financially sound and
     reputable insurance companies, on commercially reasonable terms, insurance
     with respect to its properties and business and the properties and business
     of its Subsidiaries of the types and in the amounts generally carried by
     similar businesses in the same industry.
 
                               DEFAULT; REMEDIES
 
     It shall be a default hereunder upon the occurrence of any of the following
(each an "Event of Default"):
 
          (a) Borrower shall default in the payment of any amount due hereunder,
     and such default shall continue for more than five (5) Business Days; or
 
          (b) Borrower shall fail to comply with any covenant contained herein,
     and such default shall not have been remedied within thirty (30) days after
     written notice thereof is received by Borrower; or
 
          (c) Borrower or any Subsidiary of Borrower (as principal or guarantor
     or other surety) shall default in the payment of any amount of principal
     of, or premium or interest on Indebtedness which is outstanding in a
     principal amount of at least $15,000,000 (other than the Indebtedness
     governed by this Note); or any event shall occur or condition shall exist
     in respect of any Indebtedness which is outstanding in a principal amount
     of at least $15,000,000 (other than the Indebtedness governed by this Note)
     or of any mortgage, indenture or other agreement relating thereto, the
     effect of which is to cause (or to permit one or more Persons to cause)
     such Indebtedness to become due and payable before its stated maturity or
     before its regularly scheduled dates of payment, and such default, event or
     condition shall continue for more than the period of grace, if any,
     specified therein and shall not have been waived; or
 
          (d) The filing by or behalf of Borrower or its general partner of a
     voluntary petition or an answer seeking reorganization, arrangement,
     readjustment of its debts or for any other relief under any bankruptcy,
     reorganization, compromise, arrangement, insolvency, readjustment of debt,
     or dissolution, liquidation, or similar act or law, state or federal, now
     or hereafter existing ("Bankruptcy Law"), or any action by Borrower or its
     general partner for, or consent or acquiescence to, the appointment of a
     receiver, trustee or other custodian of Borrower or its general partner or
     of all or a substantial part of its property; or the making by Borrower or
     its general partner of any assignment for the benefit of creditors; or the
     admission by Borrower or its general partner in writing of its inability to
     pay its debts as they become due; or
 
          (e) Any of the following: (i) the filing of any involuntary petition
     against Borrower or its general partner in bankruptcy or seeking
     reorganization, arrangement, readjustment of its debts or for any other
     relief under any Bankruptcy Law and an order for relief by a court having
     jurisdiction in the premises
<PAGE>   7
 
     shall have been issued or entered therein; (ii) any other similar relief
     shall be granted under any applicable federal or state law; (iii) a decree
     or order of a court having jurisdiction in the premises for the appointment
     of a receiver, liquidator, sequestrator, trustee or other officer having
     similar powers over Borrower or its general partner or over all or a part
     of its property shall have been entered; (iv) the involuntary appointment
     of an interim receiver, trustee or other custodian of Borrower or its
     general partner or of all or a substantial part of its property; or (v) the
     issuance of a warrant of attachment, execution or similar process against
     any substantial part of the property of Borrower or its general partner,
     and the continuance of any such event described in the foregoing clauses
     (i) -- (v), both inclusive, for 60 consecutive days unless dismissed,
     bonded to the satisfaction of the court having jurisdiction in the premises
     or discharged; or
 
          (f) A final judgment or judgments (which is or are non-appealable or
     which has not or have not been stayed pending appeal or as to which all
     rights to appeal have expired or been exhausted) shall be rendered against
     Borrower for the payment of money in excess of $15,000,000 and the same
     shall not be discharged or execution thereon stayed pending appeal within
     60 days after entry thereof, or, in the event of such a stay, such judgment
     shall not be discharged within 30 days after such stay expires.
 
Upon an Event of Default, the principal of this Note and any accrued interest
shall become, forthwith, due and payable without presentment, demand, protest or
other notice of any kind (including, without limitation, notice of intent to
accelerate), all of which are hereby waived by Borrower.
 
                                 MISCELLANEOUS
 
     The principal balance under this Note and any accrued interest thereon may
be prepaid in whole or in part at any time without premium or penalty.
 
     The Borrower and any and all endorsers, guarantors and sureties hereof
severally waive grace, demand, presentment for payment, notice of dishonor or
default or intent to accelerate, protest and notice of protest and diligence in
collecting and bringing of suit against any party hereto, and agree to all
renewals, extensions or partial payments hereon and to any release or
substitution of security herefor, in whole or in part, with or without notice,
before or after maturity.
 
     This Note shall be governed by, and construed and interpreted in accordance
with, the laws of the State of Texas and any applicable laws of the United
States of America.
 
                                          LAKEHEAD PIPE LINE COMPANY, LIMITED
                                          PARTNERSHIP
 
                                          By: Lakehead Pipe Line Company, Inc.,
                                              its general partner
 
                                              By:
 
                                              ----------------------------------
                                              Name: Mark A. Maki
                                              Title: Chief Accountant
 
                                              By:
 
                                              ----------------------------------
                                              Name: Scott R. Wilson
                                              Title: Treasurer