I haven't done a Thomas yet, but I've done a number of close quarters installations. The key is to make sure everything has enough room. No pinched wires, use shrink tubing instead of tape... that sort of thing. If everything's in there comfortably, you typically won't have a problem. Unless of course, you've wired something bass-ackward.
Also, it's helpful to attempt a readback on your programming track after each step of the installation to make sure you're doing everything right. Never just plop the thing on to full track power without checking the read back first... You've probably blown out the decoder. Replacement under warranty is pretty easy, but at this point, Thomas won't be chugging around the Christmas tree if that's the case.
Lee
