Trainwire:

I always thought the current-gathering part of the pantographs just rode on the bottom side of the overhead wire instead of hooking into it? At least that is what it always seemed like from the ground. [Linked Image]

When I was in San Francisco in 1998, I would occasionally see a trolley-bus ride off it's overhead wire. The driver would have to get out and use a long pole with a hook to get that springloaded arm back into contact with the power line. Granted the trolley arms are long enough and swiveled so it doesn't happen often...

At the end of the arm (don't know the correct term) there is a grooved roller that pushes up against and rides on the underside of the wire (and occasionally throws out a few sparks).

Does San Francisco still have trolley-buses? I know they're expanding their railed street-car network, but dunno about the trolley bus system. Most of those were pretty dilapidated (just like their gasoline powered brethren). [Linked Image]