Howdy!
I put a DH121 in a Bachmann Thomas and it works fine as long as the wheels and wipers are clean. The little guy has quite a bit of gear noise but kids don't mind.
The decoder fit (barely) in the cab. I popped the top off and connected the decoder to the little PC board (cut the other components off the board or just ditch the board.)
One of the new DZ123s would fit much better and make an easier installation I think.
Oh! You can seen Thomas in my avitar on this post.
Have fun!