Which means that proper wheel shape is part of the answer to full physics train. This animation shows exaggerate idea of how it works:
But real train wheels are much thinner and I’m not even sure we have enough numerical precision to implement this effect naively. I was planning to do something similar for motorcycle tires as rounded tires is what allows them to steer by tilting. Have to look at this later, so far it looks a bit like an overkill just to keep wheel meshes away from clipping a rail 
But I like this kind of simulations!