It's interesting to trace the route structure of the long distance trains through some selected Amtrak timetables:
At the start of Amtrak in 1971, only the Empire Builder operated; the North Coast Hiawatha was inaugurated within the first year.
01-16-72: Empire Builder (7/8) and North Coast Hiawatha (9/10) both use former NP line from Seattle to Pasco via Auburn to reach Spokane.
06-11-72: Same situation as 01-16-72
05-19-74: Empire Builder (7/8) uses ex-NP line as before.
North Coast Hiawatha(9/10) uses ex-GN line via Wenatchee.
06-15-76: Same as 05-19-74
08-30-80: Empire Builder (7/8) uses ex-NP line as before.
North Coast Hiawatha terminated a year or two earlier during Carter-era cuts.
08-31-82: Empire Builder uses ex-GN line to Seattle and ex-SP&S line to Portland.
Note: While the Empire Builder used the ex-NP line between Seattle and Pasco via Auburn, it used the ex-SP&S line between
Pasco and Spokane in one direction and the ex-NP line between Pasco and Spokane in the other direction. This is not
discernible from Amtrak timetables!