John, that is why I included the parameters of my experience. What I have further found is that for the most part no matter what the MFG's state once you get over three fixed driver axles min of O42 is best. Williams brass Crown series first started turning my head back to O-gauge. I brought the K4, L1, E6, and B6. The K4 ran perfectly on O31 but the L1 would throw its tires even though Williams stated O31 for both. The MTH RK Imperial PRR S2 is a big engine, probably their biggest fixed frame RK.
George, if he was to add a command to a Williams he be better off going to PS-2. The upgrade kit should cost about $175 at best (I have purchased all mine cost $160). This way he would not have to purchase a TMMC command system. And ultimately he would NOT have the smoke output capability.
Mike, a caveat to choosing the RK Imperial PRR S2. Not knowing your knowledge of the PRR S2 prototype I ad this. The S2 had a direct drive steam turbine thus it did not chuff but had a continuous whoosh exhaust when moving. Also only has a side rod(s) no piston rods, main rods, crossheads, and valve gears etc. So if you are looking for a lot of mechanical action and chuffs this would not be the way to go.
Ron