"...it had to do with gravity..."

I suppose that everything has to do with gravity to a greater or lesser degree. It's a heavy topic. But gravity plays no special role in this brainteaser other than keep the railroad cars on the face of the Earth.

"...friend Slugger works up north in the locomotive sheds setting wheel alignment. He uses a real BFH..."

With a name like Slugger, what else would he use and what else would you expect?

"...sort of like the same reason that motorcycles only tip over when they stop?"

Motorcycles tip over when they stop because the driver cannot make the tiny corrections needed to keep the machine upright. That's similar to why airplanes fall outta the air when they stop moving.

No, the answer to this question is much more in the realm of mechanical engineering, is very logical, and once (properly) explained is easily understood. It's not obvious, however, 'til it's made obvious.

When you're ready, I can give you at least two very important factoids. Here's number zero: The flanges have nothing to do with it.

Speaking of flanges, do you know what part of the train is always going backwards?

(Gee, lemme think for a second......)