The new HO Roundhouse Mogul with smaller drivers and the boiler lowered. The frame has been narrowed to Sn3 width.
Markits makes a self-quartering Sn3 "Romford Style" axle and several UK manufacturers make wheels to fit them. These are ScaleLink 14mm (36" diameter in S scale) drivers.
If you are here you have moved to the really dark, dark side of modeling, Sn3 makes you an outcast in the general world of trains. Using HO models will even make you shunned in Sn3.