I have two Williams engines: an SD-90 and a Hudson.  The Hudson runs okay on my MTH switches.  The SD-90 does not.  The SD-90 will stop just before leaving the switch unless it is running pretty fast (~8 volts or more) and "coasts" through the switch and the electrical connection makes on the track past the switch.  It seems like it could be a design of the reverser board?  When a slight power blip happens crossing the switch, it takes that to be a reverse button?  I have checked voltage in all the "live" sections of the switch and all read normal.