Iain,
The link Paul posted was a question I asked regarding had any electric Traction Engines been built, you will see from the answers that quite a few had been built and some to quite a large scale.
Although mine is on hold at the moment it is not forgotten I will get back to it as I will with one of Jasons engine designs that is 3/4,s finished.
Mine is a homebrew but based on the 1" Minnie though I am going with a scale of 1.625" scale which suits me size wise. I plan on going with a 350 watt motor mounted in the firebox and the batteries placed in the driving trolley. The drive will be through the side wall of the firebox with one additional gear connecting into the Minnies standard drive train, the low gearing is well suited to electric and should give good torque well at least thats my thinking at the moment.
Ron