As I think about it, I'm not entirely clear how you'd do a system with just one wire in the center of the street. If two trolleybusses came down the street in opposite directions, wouldn't their trolley poles interfere with each other when they crossed? A normal tram system has one wire over each track, so only one tram will be using any given piece of wire at any one time, which avoids the interference problem.
Oh, and speaking of worrying about the wires being not high enough for George's big busses: