Actually train stability is way more subtle than that. Even the "stable" one has a critical speed at which it will become unstable. This is called Klingel motion or "hunting". It is caused by the effective damping from the contact reducing as train speed increases.

https://en.wikipedia.org/wiki/Hunting_oscillation

Great video from 1937 on differentials: https://www.youtube.com/watch?v=yYAw79386WI

I think it's been discussed a few times on HN before. The fact that it's over 80 years old yet easier to understand than newer material is amazing.

The 6-wheel diff video starts a bit late and misses some details. They first start talking about 6-wheel diffs here: https://youtu.be/LwZBnMQ40rI?t=248

The question about train wheels reminds me of one of my all-time favourite videos, where Feynman explains how trains follow their tracks: https://youtu.be/q5PAJ4tEIsM

The answer to all your questions is "with a nonzero but acceptable amount of scrub" ;)

Caster and trail are interesting concepts which make it so the steering automatically returns to straight. It is confusing that the casters on office chairs use trail, not caster. It should be intuitive why one can ride a bike hands free, but good luck on a bird/lime.

Imagine those Soviet icbm trucks with two front turning axles and three rear axles. Or even worse the MZKT 79221[1] which was like eight axles.

[1]https://en.m.wikipedia.org/wiki/Missile_vehicle