Thanks for all the great input, guys. The trolley I'm using is a 2005 MTH RailKing 30-2581 NFL Steelers Bump-n-Go Trolley. It's pretty basic - no Protosound. It really has to have 0 volts to stop, and I'm afraid the poor passengers are going to have to get off the trolley in the dark! I think I'll get one of the relay circuit boards and then just play around with it, trying some of what y'all suggested. For about 4 bucks, even if I can't get it to work the way I want, I'm not really out much...
Dale