Small progress update
Tried some different gearing on the tank this weekend, using Mecanno cogs to double the input speed to the gearbox, the idea being to make the tank twice as quick.
Sadly, there's not enough torque in the engine to make smooth turns with this arrangement, so I've decided to revert to the original 1:1 engine to gearbox ratio. No Speedy Gonzales for me, by the looks of things.
Also, I've noticed that there's quite a lot of oil in the steam exhaust, so I'm going to fit a Condenser to clean up the steam output.