Why not go for DCC? DCC typically uses 19v AC.
Run either the centre, or the common outer rail as one feed, the other as the return and let the O gauge only, outer rail look after itself.
Regauge the On30 locomotives and stock to suit. I don’t know about in the USA, but O14 is a thing in the UK, with specialist trade support for 14mm Gauge axles and wheelsets.