If your problem needs a complete fix, you can find the whole motor housing gears and all sometimes on Ebay UK. Then all you need to do is switch out the whole unit which is very easy. I have bought a few from ambers-alcove2008 in the past. It doesn't look like she has any listed now but maybe she has more coming. She is very nice and it may be worth it to drop her a note. There may be other sellers in the UK that sell these that I am not aware of to. If you do manage to find the whole replacement unit, don't through away the old one. Use it to practice on disassembling and reassembling it and save all the working parts for future repairs on other trains.
You may want to check this thread out for some information. http://www.blueplastictracks.com/showthread.php?tid=242
You may want to check this thread out for some information. http://www.blueplastictracks.com/showthread.php?tid=242