Subway stations

Last updated: April 8, 2006.

Most of the world's major cities have a subway, a compact electric railroad running underground that carries hundreds of millions of people each year. Subway stations are laid out to channel crowds of people off the street, through ticket halls, underground to where the trains run, and on and off the cars themselves. Flows of people traveling in different directions are carefully separated and special escape routes are provided for emergencies.

Photo: Energy-saving lamps in a subway station in Stockholm, Sweden. Photo by Lawrence Berkeley National Laboratory courtesy of US Department of Energy.

Subways of the world

If subways were engineering novelties at the start of the 20th century, they are major urban transportation systems at the start of the 21st. The New York City subway opened with 28 stations in 1904 and has over sixteen times as many (468 stations) today. 60 percent of these stations are underground and the deepest of them, 191st Street in Manhattan, is 180 ft (55m) below the surface. Passenger numbers have grown too. Within days of its opening, a million people were trying to use the New York City subway and today it carries over a billion people each year. But the world's busiest subway, in Moscow, carries over three times as many people.

Photo: Around 7 million people use the Moscow subway each day. Aerial view of Prospect Mira station by Don S. Montgomery courtesy of DefenseImagery.mil.

How subways are built

Subways present major challenges for architects and engineers. Stations and track must be constructed underground, often without disrupting life in the city streets above. Typically this is done by sinking deep vertical shafts into the ground and excavating horizontally beneath existing buildings. Deep tunnels are constructed by mole-like machines that bore through Earth piling up spoil (excavated material) behind them. Shallow tunnels are constructed by "cut-and-cover," which involves digging a trench, reinforcing the sides, then adding a roof on top. Once excavated, stations are fitted out with ticketing halls, escalators, emergency stairs, and platforms.

Photo: Subway tunnels are reinforced with prefabricated steel and concrete to stop them collapsing inward.
Photo by Mickey Sanborn courtesy of DefenseImagery.mil.

Ticket hall

A subway station funnels people off the street into a large ticket hall, where automated barriers check the validity of their tickets. Heavy-duty escalators (sometimes stacked in several flights) carry people underground to the train platforms and electric platform edge doors protect people from falling accidentally onto the track. A subway station is often laid out symmetrically so that one side guides departing passengers downward, while the other side guides arriving passengers upward.

How an escalator works

An escalator is a people conveyor in which a drive chain carries a series of metal steps around in an endless loop. Hidden wheels at the front and back of each step run on inner and outer rails. The changing spacing between these rails causes the steps to fold in and out as they cycle around. A high-speed electric motor and a system of gears turn the drive chain slowly but with considerable power. A typical escalator turns at a speed of roughly 150 ft/min (46 m/min) and can move 10,000 people in an hour.

Photo: How to move while you're standing still—the brilliantly simple escalator. Photo PH2 R. Hamilton courtesy of US Navy and DefenseImagery.mil.

Platform edge doors

Modern subway stations have electric sliding doors that maintain a toughened glass safety barrier between the platform edge and the track. Normally closed, these platform edge doors open only when a train pulls alongside. A signal from an automatic control system inside the train triggers a sensor on the platform causing the platform edge doors to open with those of the train. Although they are fully automatic, the doors can also be opened manually from the platform in an emergency. As well as increasing safety, platform edge doors reduce noise, vibration, and dust.