Many of us high rail S gaugers are running two rail TMCC and Legacy. I have around 25 conventional DC motored engines that were converted to TMCC/Railsounds. The steam engines have an added fan driven smoke unit (from MTH) and retrofitted electrocouplers. Some diesels have added electrocouplers. The conversions to TMCC are only complicated by the tight space in the S gauge shells.
I frequently run my Legacy and TMCC engines on AC powered conventional track, they work fine. Many Legacy and TMCC sounds, Tower Comm in neutral, crew Talk in F or R, are accessible with whistle and bell buttons. The engines sequence N-F-N-R if Legacy and F-N-R-N if TMCC. The coil couplers on the engines close better than most of the ones on the cars, but that is likely unique to S.