Lionel cars with articulated couplers.

You didn’t state what kind of car is behind it and derailing.  On tight radius curves you need a car with a lot of swing in the coupler, like most any postwar freight car.  The other potential problem is that the coupler on the S-1 is stiff.

Dan - So far anything behind it has not been happy...

Jeff - I had the same problem at one time and so coincidentally I put one of my heaviest Lionel cars (it has a lot of heft) right behind the tender and the problem was resolved.

FYI (in case you want to call it up on Lionel's website) the car in question is articulated , operates on 031 curves, and is described as 'PRR 16-Wheel Depressed Center Flatcar with Transformer' (6-21644). 

Good Luck in resolving the problem!

I like the idea of using a post war car. You could even replace one truck with one from a modern caboose with a long coupler. Much less expensive than buying a 100 dollar car with no garrauntee it will solve the problem.

Pete

PS Jeff - I see that one of these cars is currently listed on 'da bay.' 

Jeff, if everything you have is unhappy, I would look at the couplers on the S-1.  You should be able to push them from side to side with very little resistance other than the centering spring.  Try PH1975's suggestion and put some sort of weight in and car you have and see what happens.