Al,
I have 4 of what I think were the first run of Comet/Horizon cars that Atlas made and also had a problem similar to yours. It was not only on the last car but some of the intermediate cars would jump as they went through my Ross turnouts. You did not state whose track and turnouts you use so I don't know what I will say will apply. First off, I can understand that the last truck in line is the offender. All the other trucks are pulled through the turnout and have tension on the couplers that are attached to the trucks. The last truck has no tension on the coupler which makes it more susceptible to derailing. As you said, no matter what car you put at the back, the last truck is the one that derails. The fix that I made will be listed a little later in this post. You will also notice that the pick-up roller travels straight up and down. It is not mounted on a angled arm as is your locomotive rollers. Besides this up and down movement, there should be some degree of movement both front and back in a rocking motion. This is where I found the problem in my case. The rocking motion was very limited and I found that that the pick-up roller itself was dropping down into the gap of my turnouts. When it was time for the roller to be forced up as the gap got smaller and due to the roller not be able to rock, it would "lift" not only the roller up but the whole truck. With no tension on the truck from the coupler, it was now free to turn and derail.
Sorry to say, but you are going to have to take the car apart for the fix. Not a hard job though. Once the chassis is removed, you will find the blue plastic floor of the car. On both the front and rear trucks you will find a crescent shaped slot where the hot wire for the pick-up roller has clearance to mate with the tab on the roller assembly. You should find that the wire connector is either rubbing or very close to the edges of the slot in the plastic floor. This is preventing the roller from rocking back and forth. I took a dremel tool and widened the slot (this will not affect the appearance of the floor from outside the car). Grind away until the pick-up is able to rock back and forth without coming in contact with the edges of the slot.
To see if this is your problem, you might be able to temporarily fasten the pick-up roller in the up position so that it is not in contact with the middle rail. I would try some tape to do this. Run the car through the turnout in this manner and see if the final truck now makes it through without derailing. If it does, I think my fix should cure your problem. Just to let you know, I did this fix to every car and every truck and have had no more problems.
