Britannias & Derby design

Unless they are hinting at, within a loco of about the same length, a 2-8-2 would have smaller driving wheels than a 4-6-2? So at a given speed, more revolutions per minute, so more piston strokes, so more steam consumed per given time - i.e. needing a greater rate of steam production to achieve a given speed.

I admit I'm slightly clutching at straws here...

Edit: but I don't think that necessarily makes sense as at the same speed and weight of train, and ignoring any minor variation in internal resistance, the power required should be the same either way, so same steam consumption needed. So smaller wheels should require more strokes per minute to do the same speed, but also less steam admitted per stroke - so the two effects should counterbalance.

Tom

 

Clearly someone feels that unless you can get the thing to slip at the drop of a hat theres a problem, but again thats not a boiler size issue more a boiler pressure/cylinder size one...