A now almost unique current collection system used by London Underground which has separate positive and negative current rails. The same system was used by the LNWR and the Mersey Railway at one time. The usual 3-rail method of connecting return current via the running rails is replaced by a fully insulated system using separate positive and negative rails. Originally used to reduce the risk of stray currents causing damage to nearby utilities and structures through electrolysis. Has the disadvantage of requiring special fault detection as earth faults do not cause current to automatically switch off. See conductor rail.