FWIW, I am involved in railcar maintenance and this is a pretty common problem. One or more of the sensor output voltages is dropping too low to detect at low speed and the system sees that as a zero speed wheel. Most likely it is caused by the magnetic dust on the sensor reducing its output. The system works by detecting frequency of course, but if the voltage is too low, it won't read it. At higher speeds the voltage goes above the threshhold and the system works fine.