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 motio

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