Spokane switching westbound is the easiest thing in the world: Combined train stops alongside the platform. Lead unit pulls ahead and out of the way. Uncouple just ahead of the lounge car. The former 2nd unit remains in place, becomes the Seattle section's engine, and departs. The original lead unit backs onto the lounge car and departs for Portland ~20 minutes later. HEP is off for 10ish minutes in the Portland section. Eastbound I have been asleep. Portland section still arrives first, and the end result is still the Portland engine being on the point after departure. But it must require at least one extra move-- I don't know if that extra move is the Seattle section going past the parked Portland section and backing into the platform ahead of it, or something else. Nor do I know if the Portland engine "hides" east or west of the station while waiting for the Seattle section to pass it. I doubt that the combined Builder has ever *needed* 3 units. It routinely ran with 2 F40s in the 80s and early 90s, and routinely runs with 2 P42s now, summer and winter. In seven trips and several dozen sightings, the only time I've ever seen a 3rd unit, it was a BNSF freight unit leading a 4-hours-late westbound, presumably after a failure somewhere west of Spokane. There was a gap of several years when I neither saw nor rode it -- approximately 2005-2013. Were there times when the Seattle section was heavy enough to need 2 units? Do not know. If it had 2 seattle sleepers and 3 seattle coaches, perhaps it would. I don't recall ever seeing 3 seattle coaches, either - it would be a rarity to need them, since there are always 2 portland coaches which tend to have lots of extra capacity, and anyone not actually going past Spokane on the Seattle section is likely to get placed in them.