Which metro system uses a unique four-rail electrification system?

The London Underground uses a distinctive four-rail electrification system.

Two ordinary running rails carry the trains, while a third rail beside the track supplies positive current and a fourth rail positioned between the running rails carries the negative return. On much of the deep-level network, the nominal arrangement is about +420 volts and −210 volts, producing a 630-volt supply. Some subsurface sections have been upgraded to 750 volts.

The design was adopted in the early twentieth century as London’s underground lines were converted from steam to electric operation. Keeping the return current off the running rails reduced leakage into cast-iron tunnel linings and nearby metal utility pipes, helping prevent electrolytic corrosion. It also left the running rails available for track-circuit signalling.

“Unique” here means unusually distinctive rather than literally the only four-rail railway ever built. Early Milan Metro Line 1 also used a four-rail arrangement, but London’s system is the famous surviving example. This is also why the London Underground should not be confused with most third-rail metro systems, where the running rails commonly carry the electrical return.