Most of our DC sources are left floating. Of course our biggest source, traction power, has the negative return at a near ground potential. Since the return path is also part of the supporting structure of trucks wheels and running rails, it would be very hard and dangerous to elevate it well above ground. Since touching the third rail is as dangerous as messing with Social Security, it is kept elevated and isolated on stand-offs called frogs. If one of these frogs gets fried, it will not taste like chicken.
Joe
