Amtrak law
I don’t know if this will help you win your case, but it is all the Oregon law I could find on this. You could argue that your situation is different than the mechanic’s in the case, because your trips are part of your regularly assigned duties. The regulation is the definition that is binding on the Revenue Department. The material is in the CCH Oregon State Tax Reporter, which may be at your local law library.
Luther G. Butler and Dorothy L. Butler v. Department of Revenue.
Oregon Tax Court, No. 3873, June 27, 1997
A Washington resident who was a mechanic for an interstate trucking company was not exempt from Oregon personal income taxes under the Amtrak Reauthorization and Improvement Act of 1990 (P.L. 101-322), because he did not, as an employee, perform regularly assigned duties in two or more states. Although the employee performed services at the company's Portland terminal and traveled outside the state about three times a year to pick up parts in emergency situations under a company policy requiring employees to keep company trucks operating in a safe condition, the trips to pick up parts were not part of his regularly assigned duties, which were to inspect and repair trucks.
Oregon Regulation, OAR 150-316.127-(E).
(1) General: Various federal laws affect the application of Oregon tax laws to nonresident employees of motor carriers, rail carriers, air carriers and water carriers. For purposes of this rule the following definitions apply:
(c) "Regularly assigned duties in more than one state" means duties that are performed on a regular basis in more than one state, e.g., daily, weekly, or monthly assignment. Duties that are performed on an "on-call" or "as-needed" basis, or duties that are performed on a sporadic or intermittent basis during the year, are not considered to be "regularly assigned duties."