Hi Walt,
Will follow along on this in the hopes of learning a little more about why cars derail when they shouldn't as in your case here. There is nothing less fun on a layout than having the nice consist you put together fall apart in route when some wheel set decides to climb over the rails. 
I would think that the tender is nice and heavy so next maybe look at wheel spacing. I've had some cars where the wheel spacing is wrong for whatever reason. Also maybe look at the truck and make sure it is rotating freely on it's pivot point. And maybe even the car right behind the tender may be causing the issue. Good luck.
