The New York City Subway primarily uses a third rail to collect electric power.
A third rail is an additional conductor placed beside the running rails. A train’s contact shoe touches this energized rail and draws direct current for its traction motors. The arrangement avoids overhead wires, which is useful in tunnels and on many elevated structures.
The subway’s standard traction supply is 600 volts direct current. The system uses different third-rail arrangements in places, but the basic principle is shared across the historical IRT, BMT, and IND divisions.
The Staten Island Railway also uses third-rail electrification, although it is operationally distinct from the New York City Subway. The Long Island Rail Road, by contrast, uses third rail too, while many other railways use overhead wires. Third rails are hazardous, so passengers are instructed never to enter track areas.