Upon opening a session, I issue a Drive to Trackmark command to a locomotive, which previously worked fine.  However now the locomotive just sits there, and the bogeys or wheels disappear!  I can't even move it manually.  Now every time I start a session on that layout it does the same thing, with any locomotive and any trackmark.  Even if I delete the session and start a new session with new rolling stock.
Any hints what it could be?
				
			Any hints what it could be?