This is how I make a 2 rail atlas 148 turnout operate with no shorting when converted to three rail.
First, one has to flip the turnout over and cut any inter rail electrical connections including the switch rails and frog. All must be dead.
Second, negative power goes only to the outer rails only. The loco doesn't care which side is the negative or ground.
Third, Positive power obviously goes only to the center rails.
Switch rails gapped electrically so even though they pick up negative current through the outermost rails, the current is not passed through to the crossing rails or frog.
All interconnections are severed.
This would be much easier to do with hand built turnouts. If you have any negative current flowing through the inner rails the locomotive or car rollers will spark or short as the train transitions through the inner rails.
I hope this is clear. Also, you can see how I relocated the guard rails to permit the wider flanges of 3 rail locos to go through.
