A subway moves passengers through underground tunnels using electric trains that draw power from a dedicated rail, follow precise signals to maintain safe distances, and rely on a network of infrastructure you never see: power substations, ventilation shafts, and maintenance crews working through the night. The basic concept is straightforward, but the engineering behind it involves dozens of interlocking systems that keep everything running safely at scale.
How Trains Get Their Power
Subway trains run on electricity, but they don’t plug into anything. Instead, they pick up power from a third rail, an electrified rail that runs parallel to the two rails the train’s wheels ride on. A metal contact shoe on the underside of each train car slides along this third rail, drawing current continuously as the train moves.
That electricity starts its journey as ordinary high-voltage alternating current (AC) from the local power grid, the same grid that powers homes and businesses. But train motors need direct current (DC) to operate, so the power goes through a multi-step conversion process before it ever reaches the tracks. On the Bay Area Rapid Transit system, for example, incoming power from the utility arrives at roughly 60,000 to 115,000 volts AC. A high-voltage substation steps that down to 34,500 volts AC for distribution. Then, at traction power substations spaced along the route, transformers reduce the voltage further and rectifiers convert it from AC to DC. The final product is 1,000 volts of DC power fed into the third rail. BART alone uses 72 of these traction power substations and about 225 circuit-miles of distribution cable to keep its trains running.
Some newer systems, particularly light metro lines, use an overhead wire (called a catenary) instead of a third rail. The principle is identical: the train collects DC power through a contact point as it moves. Third rails are more common underground because they fit in tight tunnels without requiring extra overhead clearance.
What Makes Trains Move and Stop
Once electricity reaches the train, it powers electric traction motors mounted on the axles. These motors convert electrical energy into rotational force that spins the wheels. Modern subway cars use AC motors controlled by onboard computers that adjust speed smoothly, which is why you feel a gradual acceleration rather than a sudden jolt.
Braking works in reverse. When a train needs to slow down, its motors switch into generator mode, converting the train’s momentum back into electrical energy. This is called regenerative braking, and on many systems, that recaptured energy feeds back into the third rail for other trains to use. Only at very low speeds do traditional friction brakes take over to bring the train to a complete stop. This dual system saves a significant amount of energy across a busy network and reduces wear on brake pads.
Signaling: The Subway’s Nervous System
The signal system is what keeps trains from colliding. It controls when trains can move, how fast they should go, and which direction track switches will route them. Without it, running dozens of trains through the same tunnels would be impossible.
Older subway systems use a method called fixed-block signaling. The track is divided into sections, some over 1,000 feet long. Electrical circuits in the rails detect whether any part of a train is somewhere within a given block. If a block is occupied, the signal behind it turns red and the following train must stop or slow down. The limitation is precision: the system knows a train is somewhere in the block, but not exactly where or how fast it’s going. To compensate, trains have to maintain large safety buffers, which limits how many trains can run on a line at once.
Newer systems use Communications-Based Train Control (CBTC), which replaces those rough estimates with continuous, precise tracking. Each train carries onboard equipment that communicates wirelessly with wayside devices along the track, which relay data back to a central operations center. The system knows every train’s exact position and speed at all times. Because that information is so precise, trains can safely run much closer together. The result is shorter wait times, less crowding, and the ability to run trains at higher speeds without compromising safety. Upgrading from fixed-block to CBTC is one of the biggest infrastructure investments subway agencies undertake, requiring new equipment on every train, along the tracks, and in control centers.
How Tunnels Are Built
Subway tunnels are constructed using two primary methods, and the choice depends mainly on how deep the tunnel needs to be.
Cut-and-cover is the simpler approach, used for shallow tunnels. Workers dig a trench from the surface, build the tunnel structure inside it, then cover it back up and restore the street above. Many of the world’s oldest subway lines were built this way, including much of the original New York and London systems. The major downside is disruption: streets are torn up for months or years, and buildings along the route may need to be purchased and demolished. It’s effective but messy.
For deeper tunnels, transit agencies use tunnel boring machines (TBMs), massive cylindrical machines that grind through rock and soil while simultaneously installing precast concrete lining segments behind them. TBMs work far below the surface, so they avoid tearing up streets and don’t require buying surface property. That said, they’re expensive. City subway TBM tunnels cost roughly 500 million euros per kilometer as of recent estimates. The tradeoff is speed and minimal surface disruption. London’s Underground switched from cut-and-cover to bored tunnels toward the end of the 19th century precisely because the surface chaos had become unacceptable.
For tunnels at intermediate depths, either method can work, and the decision often comes down to geology, budget, and how much disruption the city is willing to tolerate.
Track Gauge and Rail Standards
Most subway systems worldwide use standard gauge track: rails set 4 feet 8.5 inches (1,435 mm) apart. This measurement dates back to George Stephenson’s Liverpool and Manchester Railway in 1829 and now accounts for about three-fifths of all rail trackage globally. Using standard gauge allows subway operators to source trains and components from a wide market of manufacturers.
There are exceptions. Some systems inherited wider or narrower gauges from their national rail networks. Russia uses a 5-foot gauge, Japan’s older rail lines run on 3-foot-6-inch gauge, and a handful of countries operate on multiple gauges simultaneously. India uses four different gauges across its rail network. But for new subway construction, standard gauge is the overwhelming default.
Ventilation and Temperature Control
Underground stations and tunnels generate a surprising amount of heat. Train brakes, motors, passenger body heat, and lighting all contribute. Without ventilation, temperatures would climb quickly to uncomfortable or dangerous levels.
Subway systems use two main strategies to move air. The first is the piston effect: as a train moves through a tunnel, it pushes air ahead of it and pulls air behind it, essentially acting as a giant plunger. This natural airflow blows through entire stations and can be harnessed through carefully designed ventilation shafts. The number, size, shape, and placement of these shafts directly affect how well piston winds perform. In tropical climates, piston winds are especially valuable in summer for releasing internal heat and reducing the load on air conditioning systems.
The second strategy is mechanical ventilation: fans and HVAC systems that actively push or pull air through the network. These are essential during emergencies, particularly fires, where smoke needs to be directed away from passengers and toward exhaust points. The design of station entrances and shafts also plays a role in keeping out rain, moisture, and extreme cold from the surface. When neither mechanical ventilation nor piston winds are operating, hot air naturally rises through stairwells, elevator shafts, and vents due to what engineers call the stack effect.
Platform Screen Doors
Many modern stations install glass barriers between the platform and the tracks that open only when a train is stopped and its doors are aligned. These platform screen doors serve multiple purposes. Full-height versions seal the station from the tunnel, dramatically improving climate control by keeping conditioned air in the station and tunnel heat out. Shorter half-height gates cost less and still allow additional ventilation in stations that don’t have climate control.
The safety impact is striking. Research compiled by the Federal Railroad Administration found that installing platform screen doors reduced railway injuries by roughly 69%, decreased suicides by 59% to 89% depending on the study, and eliminated accidental falls entirely at equipped stations. They prevent people from accidentally or intentionally entering the track area, which is one of the leading causes of subway fatalities worldwide.
Keeping the Rails Safe
Subway tracks take an enormous beating. Thousands of train passes per day cause wear, tiny cracks, and surface defects that can grow into serious problems if left unchecked. Maintenance crews use several detection methods to catch issues before they become dangerous.
Ultrasonic testing sends high-frequency sound waves into the rail to find internal flaws invisible to the eye. Magnetic flux leakage testing detects surface and near-surface cracks by measuring disturbances in a magnetic field applied to the steel. Visual inspection now uses 2D and 3D imaging to assess rail surface condition in detail. The standard approach combines all three: magnetic and visual methods catch shallow defects before they develop into dangerous transverse cracks, while ultrasonic testing finds deep internal flaws that could lead to a rail breaking under load.
Specialized grinding trains also run periodically to restore the rail’s profile, shaving off surface irregularities and extending rail life. Most of this work happens during overnight shutdowns, which is why many subway systems close for a few hours each night rather than running 24 hours.