Trains in tubes have a lot of suction that draws in condensation from outside air.
A curious advantage of instrumenting all axles for wheel turn counting is calibration against actual known distances (end to end, etc) over time reveals slow wheel degradation that can trigger carriage maintainance.
Not all operators do this, it's been done at various times for heavy rail mining operators with loooong trains and heavy daily usage.
Also trains have quite a large number of axles, I'm sure you could reduce error by counting all of them.