Some lines use two power rails, such as the London Underground. This could be made explicit by electrified=dual_rail. Sometimes one rail is grounded and the other rail has a positive voltage; sometimes both rails have (different) voltages. On the London Underground the centre rail is at -210V and the outside rail is at +420V (see London Undergound). Shall we tag this as voltage=-210;420 ? Some (above-ground) parts of the system have the centre rail at 0V and the side rail at the positive supply voltage, to allow track-sharing between London Underground 4-rail trains and normal 3-rail trains.
More information on Wikipedia.