Interesting that this thread raises its head again just as someone requests info on how to start with a Garrett. Looking again at the Stainz conversions, while they are well done and very pretty, a straight Stainz conversion using the boiler does create a locomotive that in real life would barely manage to pull itself. I think a new chunkier boiler would make a better bet and my suggestion would be to consider a Newquida Boiler on those chassis. I think that just this change would make a more believable machine.
