Richard, sorry to say, but if I understand the situation correctly, nobody has come up with the solution to your problem yet.
Let me restate the problem as I understand it. You have a switch in your trolley route. You want the trolley to use leg "A", then return and use leg "B" on the next trip. Repeat A, B, A, B...
This is a more complicated operational pattern than simple solutions can handle. Short of an electronic solution, I think it can be done with a couple relays and three insulated sections, one located on the A and B legs and one on the common leg.
Here's the logic: If A then B next, if B then A next, where A and B are the route last taken.
The way this works is one relay keeps track of A or B and the other relay throws the switch and latches or unlatches the first relay. Latching is when the power to the relay's coil is run through a set of the relay's contacts, thus holding it in the on condition. It doesn't matter which way it goes, but let's say B is latched and A is unlatched. This latching and unlatching is controlled by the insulated sections on the A and B legs. The third insulated section, on the common leg, would then tell the switch to throw in the direction indicated by the first relay, and reset the first relay to the next condition.
Sorry Tom, but a spring switch isn't going to work for this one, as there is no loop in the track plan. The shape is a "Y".