Hi James, I have a couple of Brian Caton's Live Steam loco's in 009 (same scale as 00 but for narrow gauge) and I have good knowledge of how his 00 locos worked. I was lucky enough to be a good friend of Brian, and he was my mentor in model engineering for many years. he was a superb model engineer with an encyclopedic knowledge of the railways.
The 00 locos had a refillable gas tank located in the tender and a gas fired centre flue boiler mounted in the traditional place. The locos were driven by a single acting piston valve engine with slip eccentric mounted in the cab. The crank of the engine was running parallel to the loco and then gear reduced down and linked to one of the driving axles of the loco. From there the coupling rods of the model did their own work. The engines were not self starting but after a gentle push in either direction the engine would run happily, in forward or reverse, due to the slip eccentric. the locos were fitted with a regulator which was servo controlled through the track, this could be isolated in order to not play havoc with 'foreign' layouts. The regulator could also be controlled from the cab by hand, if you were delicate.
I'd be very happy to answer any questions you have on his loco's I can probably find a few pictures of them.
Regards,
Will