The LA Red Line has been a favorite to photograph. Despite many warnings from security, I have yet to have my camera confiscated or be arrested. None of the locals have given me any trouble, though I do watch my back regardless.
It's too bad Los Angeles hasn't expanded this system further. The light-rail systems are slow, have street running and traffic to deal with. The Blue Line is reasonable... The Gold Line that opened a few years ago is way too slow! Capacity on the Red Line is awesome, frequencies are real good, and the trains arrive and depart very quickly.