Get some toothpicks. Start spreading the shell apart from the frame, but insert toothpicks as you go. Don’t force things, but after a few toothpicks, the two major assemblies should separate cleanly.
Ah, yes, the old toothpick thing. I seem to remember having to do that with a ConCor Electro-Liner.
Now if only the so-called “HO scale” people would fit. The figurines from China seem to be a tad too large. I did some on an O scale Aerotrain last week - same deal -heads against the ceiling.
I found that on some of the cars I had to shave a little off the truck bolsters in order to allow the trucks to rock a little more. The clip-in tabs were too tight.