Ok. So here's how I think the module works.
As long as there is a signal being received at its input, via the track rails (it doesn't matter if the input signal is bi-polar or uni-polar), the output will be ON causing the module output voltage to be low. This would be the normal "Track Unoccupied" condition.
If a train comes along and shunts the track, the input signal is blocked, the module input signal is lost, and the module output then goes high.
This is the "Track Occupied" condition.
An assumption for both of the above conditions is that the module output is pulled up.