The third rail is an electrified metal rail that runs alongside subway tracks and supplies power to the trains. It carries 600 to 750 volts of direct current, enough to be lethal on contact. While the two standard rails guide and support the train, this additional rail serves as the power source, replacing the overhead wires you might see on commuter trains or streetcars.
How the Third Rail Powers a Train
The third rail sits either next to or between the two running rails, raised on insulated supports to keep it from grounding out. A metal device called a contact shoe is mounted on the underside of each train car and presses against the rail as the train moves. This shoe picks up electricity from the rail and feeds it to the train’s electric motors. The circuit completes through the running rails themselves, which carry the current back to the power substation.
Third rail systems operate at 600 to 750 volts DC, which is relatively low compared to overhead wire systems that use 1,500 to 3,000 volts. The tradeoff is that lower voltage means higher current to deliver the same power, which leads to greater energy loss during transmission. Substations need to be spaced closer together along the line to compensate.
What the Rail Is Made Of
Modern third rails are typically composite structures designed to balance conductivity, weight, and durability. A common design uses a hollow aluminum extrusion for the main body, since aluminum conducts electricity well and is much lighter than solid steel. The top surface, where the contact shoe slides along, gets capped with a strip of stainless steel that resists the constant abrasion of metal-on-metal contact. Older systems used solid steel rails, which are heavier and less conductive but extremely durable.
Three Ways to Make Contact
Not every third rail system works the same way. The differences come down to where the contact shoe touches the rail.
- Top-contact: The shoe rides along the top surface of the rail. This is the most common and simplest design, used in systems like the Toronto subway.
- Bottom-contact: The shoe presses upward against the underside of the rail. This design has a built-in safety advantage because the energized surface faces downward, making accidental contact less likely. Metro-North in New York, Philadelphia’s Market-Frankford Line, and London’s Docklands Light Railway all use this configuration.
- Side-contact: The shoe presses against the side of the rail. The Hamburg S-Bahn has used this approach at 1,200 volts DC since 1939.
Why Subways Use It Instead of Overhead Wires
The main reason subways favor third rail over overhead wires is clearance. Underground tunnels are expensive to dig, and every inch of height matters. Stringing overhead wires requires extra vertical space for the wire itself, the support structures, and the pantograph (the folding arm on top of a train that reaches up to touch the wire). A third rail eliminates all of that, allowing tunnels to be smaller and cheaper to build.
The system has real drawbacks, though. That electrified rail sitting at ground level is a serious hazard for track workers, emergency responders, and anyone who ends up on the tracks. Overhead wires are dangerous too, but they’re at least out of reach. Connecticut actually banned unprotected third rails back in 1905 after concerns about livestock being electrocuted. Third rail also limits train speed, which is why high-speed rail lines always use overhead catenary systems. And unlike overhead wires that provide continuous power, third rail has gaps at switches and crossings where the train briefly loses contact.
Safety Covers and Protections
To reduce the risk of accidental contact, transit agencies install protection boards on top of the third rail. These covers are typically made from fiberglass, plastic, or timber, all non-conductive materials that create a physical barrier between the energized rail and anything that might fall or step on it. The contact shoe slides underneath these covers to reach the rail surface.
The protection boards help, but they are not foolproof. A study of 16 electrical injuries from New York’s 600-volt subway third rail found high rates of serious burns, surgical procedures, and complications. Among nine non-occupational victims (people who weren’t track workers), three died. The injuries came from direct contact through falls, suicide attempts, or risk-taking behavior. Even at 600 volts, well below the traditional “high voltage” threshold of 1,000 volts, the current is more than enough to kill. For context, household current in North America runs at 120 volts, and the third rail carries five to six times that.
How the Rail Is De-Energized in Emergencies
When someone falls onto the tracks or a train breaks down, the third rail needs to be shut off quickly. Transit systems are divided into sections, each fed by its own substation, so power can be cut to a specific stretch of track without shutting down the entire line. A central control room can remotely de-energize any section. Many systems also have local emergency trip stations along the trackway that allow workers or first responders to cut power on-site.
Once power is reportedly off, crews use verification devices to confirm the rail is actually de-energized before anyone approaches it. They may also use “shoe lifts,” mechanical devices that physically raise the contact shoe away from the rail on a disabled train, preventing it from accidentally drawing power if the section is re-energized.
Cold Weather Challenges
Snow and ice are persistent enemies of third rail systems. Ice buildup on the rail surface prevents the contact shoe from making a clean electrical connection, which can stall trains or cause power interruptions. Transit agencies fight this with several strategies. Scrapers and brushes, sometimes mounted on dedicated maintenance trains, physically clear ice from the rail surface. Some systems use heated elements along the rail to keep the temperature above freezing. High-powered air blowers can blast snow and ice off the rail and surrounding switches at air speeds up to 860 kilometers per hour.
Heating systems are effective but energy-intensive and expensive to run across an entire network, so most agencies reserve them for critical points like switches and station areas where reliable contact matters most.
A Brief History
When engineers designed New York City’s first subway, the Interborough Rapid Transit system that opened on October 27, 1904, they rejected overhead wires as impractical for underground operation. Instead, they adopted a 600-volt direct-current third rail system, one of the first large-scale applications in the United States. The design drew on earlier innovations, including Granville T. Woods’ 1899 patent for a track-to-motor circuit that informed the design of the contact shoe. The basic principle has remained remarkably stable for over a century: an electrified rail on the ground, a sliding shoe on the train, and a simple circuit that moves millions of people every day.