Noel, yes the chassis is from the railroad St. Paddy model. As space is a bit tight it was a matter of squeezing the weight in wherever possible.
I tried a few decoders with mine, including a Bachmann and TCS one. I damaged the built in decoder socket during construction, so decided to hardwire a cheap Hornby decoder in for testing the model; I actually found this to work the best with the 55 despite the warnings I had heard about them. This is what is fitted in the video above.