The fastest and shortest routes from Portland would all be owned by UP. But the most likely route would be BNSF out to Montana, trackage rights over MRL...drop south through Gillette and Cheyenne, Wyoming to Denver.

Or, the next possibility would be south on BNSF through central Oregon to Northern California to Keddie. Then turn east at Keddie over BNSF trackage rights through Salt Lake City, Grand Junction CO, and over the Moffat line. Moffat tunnel 4449 would likely be either towed through or pushed with low fire like they did through the Stevens Pass Cascade Tunnel a number of times when operating over the former GN. If they really wanted to make an appearance in Sacramento at the RR museum, then it would be BNSF trackage rights over Donner east.

A trip like this would likely need to be backed and supported money wise by the railroads for some special event.