If you visited the carhouse's from time to time, you can see them doing this for testing the breaking system. There is video or 2 up on my site showing this in action.
Have seen cars backing up for blocks due to problems on the line in front of it or them. During the G20 event, All Queen 501's Westbound that got caught up in the police block at Spadina (about 10??), were backup to Shaw St so they could go east to the yard.
The current power poles make reversing quite difficult, esp. over any junctions where wires split. Apparently the new streetcars, with pantographs, will have an easier time.