Do you already have the trolley car(s) you want to use in-hand?
As suggested by others, there are several off-the-shelf "systems" that can intermittently stop, reverse, delay, etc.
Most guys on OGR want an off-the-shelf, plug-and-play solution. But if you don't mind DIY wiring, you could cobble together some timed-relay modules (a few bucks each on eBay) and get it done for, say, $10 out-of-pocket. If this applies to you, I can elaborate.
