MTH 3/2 steam locomotives are set up like 2-rail locomotives (they kinda have to be to work in 2-rail. Basically, the wheelsets are all fully insulated. What's happening is you're losing your ground. The way around that on turnouts is to put a connection to common on the closure rails of your turnouts, especially larger ones, but to prevent shorting from the pickup rollers, these connections will have to be tied to a relay (or Tortoise motor contacts.) We had to do that on a #8 curved turnout. Not really hard to do.
What kind of turnout motors are you using?
