Obviously the power lines are not like a shorted battery, as no meaningful power could be drawn if all of it is being 'burned' into a short circuit, so to speak. Therefore, since streetcars are able to draw power and run, the distribution system cannot be a short circuit. If there are no streetcars on a line, it's essentially an open circuit - each streetcar is a load, and multiple streetcars on a line are loads in parallel.
Wikipedia explains the concept of a trolley pole very simply:
When used on a trolley car or tram, i.e., a railway vehicle, a single trolley pole usually collects current from the overhead wire, and the steel rails on the tracks act as the electrical return.
I'm confident that an actual short circuit would trip a protection device at the DC distribution source and disconnect the line, leaving it unpowered until the fault was corrected. (Aside: there are 52 substations providing DC power to Toronto transit vehicles)