They use "shunt cables" on trolley wires too. Was an occasion last week when a tree broke up and fell (mostly next to but partly on) the trolley wire in downtown Sac. They shut down the power throughout downtown (shuttle buses - what schedule?), but at the work site also had what looked like a trolley pole on a trailer (have seen just heavy cables with a hook also used) to ground the wire to the track. Presumably a safety measure so if somebody did turn the power on while workers were there it would immediately short-circuit and blow the breakers.