Q:

How can I prevent my train from derailing?

I have motorized the Constitution Chase train by putting a PF train motor into the coal car. I disassembled the rest of the cars, down to the car bed, and let my kids build whatever train cars they want. This, plus a few cars from a City train let me run a train with the engine (which is a dummy), the motorized coal car, two short cars with fixed wheel trucks and two long cars with turning wheel trucks.
The problem is that sometimes this train works really well and other times it just cannot stay on the tracks at all.
The tracks are laid out on a carpeted floor, but I always put the train in the same place and it usually runs fine. But today's layout was just unworkable and I had to disassemble it and start over. The second layout works and I'm not sure what the difference is. The train rolls along and then whoops, the front wheels of the engine are turned, and the whole things falls down.
The layout itself is designed to withstand the perturbations of the train. This is accomplished by using a single piece of flex track every 1/4 turn and wherever there is a switch. This helps keep the track from shaking loose, as nothing is typically attached anywhere, except for the snap-together action of the track pieces.
Today I tried to settle down a particularly bad stretch of track by mounting it on some 10x20 bricks but that didn't really solve the problems. 
Is there a way to reliably run the train on a carpet base? Or even to just improve the reliability over what I have now, which seems somewhat random.

A:

One thing I would say from my model train experience is that shunting (pushing) is more likely to derail a train than pulling, especially at speed.
There are a few factors that influence this in models more than we see in "real life":

The force is usually transferred via the couplings rather than buffers. This means there is more chance that the force will be applied at the wrong angle, forcing the leading elements off the track
The elements are too light. In real "multiple unit" trains where the driving force is at the rear of the train, the front elements are still very heavy, which mitigates the effects of force distribution somewhat - if you can add more weight to the Locomotive build then you may have more success.

A simple confirmation of this would be to remove the Locomotive from the train and see if you get the same derailments?

