No idea. Could be because the train you're trying to control became invalid.
You can try using /c remote.call("fat", "rescan_trains") in the console to fix it. (which will disable achievements)
Or remove the mod, load and save the map and add the mod.
If it still doesn't work i will need a savegame
will do, if i cant get it to work again its no big deal, mod is very handy but i can live without it