I've just had this problem too - Bachmann 4MT 2-6-0 with a TCS M1 chip. Would go very slowly and wouldn't stop. Sometimes it shot away at full speed but again would not stop. I have a Hornby King Arthur with a Hornby decoder fitted, so I swapped them over: no problem now and both run fine - 4MT with Hornby decoder and King Arthur with TCS. I've subsequently purchased another Hornby decoder for the KA and used the TCS for an M7, and the M7 is perfectly OK with that. I don't know what the problem is with the 4MT.