Wheels slip and their diameter changes which throws out dead reckoning. Counters are also not accurate and will have cumulative error over time.

It’s complex because edge cases like this exist for simpler systems.

> their diameter changes

To be clear they are not cylinders, but cones (truncated). Do depending on how the train wobbles they will spin differently to travel the same distance even if they don't slip. So you definitely need some method of regular correction if you want accurate measurements.

I in fact meant that they wear through quite substantial diameter differences, but yes conical tread will further throw it out.

Train wheels spin. It’s steel on steel