Catenary is a system of overhead wires used to supply electrical power to a locomotive, streetcar, or light rail vehicle.

Unlike simple overhead wire, in which the uninsulated wire or cable is attached by clamps to closely spaced crosswires, themselves supported by line poles, catenery system use at least two wires. One wire, called the catenary wire or the messenger wire, is loosely hung in the shape of a mathematical catenary between line structures. A second wire is held in tension by the messenger wire, to which it is attached at frequent intervals by clamps and connecting wires.

Simple wire installation are common in light rail applications, especially on city streets, while more expensive catenery systems are especially suited to high-speed operations.

The Northeast Corridor in the United States features electrified catenary over a 600 mile distance between Boston, Massachusetts and Washington, DC, providing power for Amtrak's high speed Acela Express and other trains. Several commuter rail agencies, including MARC, SEPTA, NJ Transit, Metro-North, and Connecticut DOT's Shore Line East utilize the catenary to provide local service along the Northeast Corridor.

For a more complete discussion of overhead wiring system for railways, see overhead line.