Wfcn100 and JohnB: Thank you! Just what I was looking for. It looks like mostly Amtrak, with a few surviving railroad-scheme cars.
Jagged Ben: Yes, I've heard of, and used, Google Images. It usually returns thousands of irrelevant images, with a a few good ones mixed in. In this case, the main question was generic enough that I thought some of the passenger car fans here could probably answer it
off the tops of their heads, saving me and others a lot of search time.
That link you provided does have some interesting pictures, including the one in the top row of the Appekuny Mountain. Those observation cars went through Pasco in the early days of Amtrak, one at a time, on one-way trips to retirement. They'd been taken off the GN's major trains years earlier, and were used as lounge cars on secondary lines. Amtrak had no use for them, so they were all sold or scrapped.
From its pictures, it seems that the Empire Builder was almost entirely Amtrak by the summer of 1974, with the exceptions being due to car shortages related to the new North Coast Hiawatha. Apparently I can thank Expo 74 for the survival of variety in the Northwest trains.
Business in the Northwest was so good that summer that the Empire Builder sometimes ran two sections through Pasco between Spokane and Seattle, the only time I can remember ever seeing a train with green class lights. Even with the new SDP40Fs there was plenty of work for the old F units. Unfortunately, I was working that summer, so very seldom had a chance to take pictures.
Besides the EB and NCH, there was also a short-lived Seattle-Spokane train, through Wenatchee, solely for Expo. Northwest passenger trains were busy that summer!