Elliot, you beat me to it. I've been doing this on my modular trolley track for years!
Slight variation... I have the isolated track connected to a remote controlled relay (eBay under $5). When a kid is near the layout, I tell them to yell "go trolley go" to wake up the conductor. When they do, I hit the remote (cleverly hidden in my pocket) and the trolley goes from the station to a bump, reverses and stops back at the station where it hits the isolated track and stops.
Another variation... put the isolated track at each end of a bump n go trolley track.. the remote will let you control when it bumps and reverses, and when it stops.
I tell the kids that it only works for kids, not adults... when their parents try, the trolley doesn't move.. go figure!
GRJ, I like the DC offset idea!
