do subways have third rails?

Answer 1

Many subways have third rails, some have overhead wires, if ordinary trains run through part of them. Overhead wires though mean higher tunnels are needed so many subways, specially older ones, use third rails.

Answer 2

The third rail carries the power the same as the overhead. The current returns via the running rails except when there is a fourth rail as in London.

The "Southern Region" lines around London and South East England are a major example of "main line" trains that use the third rail system. However, most main line electrified railways use overhead current collection.

Answer 3

All the London Underground ('subway') has a third and, indeed a fourth, rail. However, the Newcastle-upon-Tyne metro in north-east England which is in subway through the city centres in is powered by overhead.