Plastic or die-cast trucks and couplers?
The key factor with plastic trucks and derailments is NOT weight... it is the mounting of the truck to the body. The rivets used to hold these trucks to the car bodies allow the truck to be quite loose and subsequently has a lot of wobble. It is this factor (along with the opening space and knuckle size) that causes derailments.
I have remounted the plastic trucks on every single car I own without exception... and I can back up a train though "S" shaped curves with my 9-inch short cars pushing a long train of much heavier cars with ZERO derailments. Believe me, it is NOT weight. The truck mounting has everything to do with this. I even tighten up the mounting with some cars that come with die cast trucks also.
Unlike a real train, where the coupler is mounted to the train car body, our toy trains have the couplers mounted to the truck. So when you are pusing a train backwards, all of the pressure is on the couplers directly. If the trucks are loosely mounted to the car body, they are going to be pushed upwards when going through a curve, thus causing a derailment. You want the trucks mounted to the body tight enough to allow the truck to swivel easily, yet not wobble. Again, I pushed a 20 car train backward, with 18 heavier cars with die cast trucks being pushed by 2 MPC era cars with my remounted trucks.... NO derailments. So weight is really not the determining factor.
The Industrial Rail cars often derail and they are very nice heavy die cast trucks, which by definition should not derail yet they do. The reason for this is the opening space inside the closed coupler is too small for many other kinds of couplers, thus causing the cars to bind when going through curves, thus causing a derailment. The trick here is to use a Dremel tool with a grinding bit and to open up the space in the coupler. Again, having done this I have also eliminated derailments here too.
