Usually signals are replaced to their most restrictive aspect by the front of the train occupying the track section beyond the signal. However, this is undesirable when there is a locomotive either assisting at the rear, or the loco is pushing the train (rather than pulling). The drivers of such trains need to know the aspect before it gets replaced to its most restrictive aspect as the signal may be out of sight before the driver can see it.
Thus last wheel replacement detects when the track section before the signal in question is clear and only replaces to its most restrictive aspect when that section is clear.
Compare to First wheel replacement .
Last edited by AndyG on 25/11/2018 at 17:41