Thanks, everyone, for your advice. I examined the cars closely this morning and found something I didn't expect.
The trucks on the old and new cars are not the same! I assumed they were similar because the side frames look alike, but the design is different and the trucks are not the same length. The space between the first and third axles on the old trucks is 2.5 "; it's 2.75" on the new. That discovery leads to another question - shouldn't the shorter trucks be better on tight curves? My experience is the opposite.
The wheels appear gauged properly, 1.25" to the outside of the wheel set. I set the cars on the roof, the wheels spun freely and the trucks rotated smoothly. The end play on the axles varied. Some seemed pretty tight to me. I rolled a car into a curve by hand in two directions. I felt more resistance from the the end with a tight axle. I'm starting to think insufficient free play might be my problem.
I didn't see any evidence of zinc rot, but a friend with the same cars had the problem. Two trucks from a five car set were defective.
Now the question is can I adjust free play without damaging the truck? I suspect the side frames are rather brittle. I wouldn't want to put much pressure on them.
The ultimate solution would be to build the newer, larger layout I've been thinking about for years. O54 curves would surely cure the problem.
John
