If you're getting on the trolleys near Suburban station they're all running on the same line.

As the trolley approachs the station look at the top of it, the number of the route its running will be on the sign top of the window. Just wait for the route number you want to show up and get on.

As others have vaguely alluded to if you want to get off using the back door, the steps are pressure sensitive, so when the trolley stops just step down and the doors will open.