@El Tejon
Interesting issue, Rick. By comparison, I have all modern Lionel on my Fastrack layout with only 036 curves. My freight cars all have sprung metal trucks and my aluminum passenger cars all have the metal trucks and couplers. There is some sideways movement of the couplers when they back up, especially with the passenger cars' longer-shanked couplers. And while they move way over to either side of the car end, the cars still move backwards without derailing. But then, aluminum passenger cars are heavier cars.
I've backed my trains up at fairly fast speeds on the mainlines (with my finger ready on the halt button), just to see if they could do it, with no derailments of either the freight or passenger trains. I even have a partial S curve on the passenger mainline and they go through it in reverse with no issues. Of course I keep the axles lubed and everything well-maintained. But no derailments from doing that. I have only one Fasttrack switch, but have never had any problems with it, whether the freight cars are going forward or backward.
I'm wondering of your track and older Postwar rolling stock would have anything to do with causing the derailments in reverse, especially on curves or through the switches. Maybe the solution would be to put some weight at the ends of the cars as SteveH suggested.
