@Yellowstone Special, your 4-car set cars are very similar to a later, seven car UP set with scale Alco PAs, 6-31712.
The latter had rubber diaphragm car ends and sprung trucks; otherwise they shared the same extrusion and construction with interiors and frames as your 6-39146.
Pity LIONEL did not offer a dome car to go with the 4-car set. It would not have been prototypical for your train, but it would have been a useful add-on for many.
Also, I'm familiar with the 6-31712 and Overland cars because I believe they are the only painted Lionel 15" aluminum cars with offset vista domes. Every other painted 15" aluminum vista dome car by Lionel has the dome centered in the roof.
These became the donor cars for my NP Noth Coast Limited and GN Empire Builder short dome cars.
