How Subways Work

By: Tracy V. Wilson

The City Hall subway station in New York City, circa 1900-1906. See amazing train videos.
The City Hall subway station in New York City, circa 1900-1906. See amazing train videos.
Photo courtesy Library of Congress

In many of the world's major cities, it would be difficult to imagine life without a subway. Signs, maps and station entrances are a pervasive part of the urban landscape in cities like Paris, London and New York. The Métropolitain, or Métro, has a station within 547 yards (500 meters) of every building in Paris. The London Underground, or the Tube, serves 275 stations throughout London. The New York City subway system has more than 450 stations packed into an area of 240 square miles (621.6 square kilometers). In some areas, it seems like the subway is everywhere you look, even though you can't see its tunnels from the street.

On top of that, these subway systems have been around so long that virtually no one can remember a time when they didn't exist. The world's oldest subway, the London Underground, opened in January 1863. The first lines of the Paris Métro opened in 1900, and the New York City subway began operation in 1904.These are some of the world's most famous subways, and it's no coincidence that they opened in roughly the same time period. They all came into existence for the same basic reason: Large numbers of people were moving to the cities, and the roads couldn't carry them all.


This influx of city dwellers started during the Industrial Revolution of the 18th and 19th centuries. During this period, numerous technological advances completely changed the way many people lived. Rather than working on farms in the country, people could earn a living working in factories in the city. People also emigrated from Europe to the United States, causing the population of New York City to grow from fewer than one million people in 1870 to almost 3.5 million in 1900 [Source: United States Census Bureau].

Because of this influx of people, cities and their transportation systems ran out of room. This was especially true in New York, where the East and Hudson Rivers kept the city from growing in any direction other than northward. Roadways and intersections in many major cities became congested and dangerous, and travel from place to place grew increasingly difficult. The state of the roads became a threat to people's safety and to the continued function of businesses and the government.

The horse-drawn omnibus was an common method of mass transit in the days before the invention of subways.
The horse-drawn omnibus was an common method of mass transit in the days before the invention of subways.
Photo courtesy Wild Horse Books & Art

At first, municipal authorities and private businesses tried to make the best of limited roadways with above-ground mass transit. Stage coaches and horse-drawn carriages called omnibuses carried groups of people from place to place. But these vehicles had to share the roads with horses, carts and the first cars. They did little to relieve the congestion, and they quickly became dirty and dangerous. Sometimes, delays and crowded conditions led to violence.

In London, city leaders eventually came to the conclusion that they'd have to build a new system of transportation to keep the city running. Since there was no space above ground, the only option was to put this system under the city. Other cities around the world followed London's lead. Today, there are more than 160 subway systems on Earth, most of which have come into existence because of overcrowding, pollution or urban sprawl.

But tunneling under a city is easier said than done. It was particularly difficult during the construction of the first subways. We'll look at exactly what it takes to dig subway tunnels and make them operable in the next section.

Subway Tunnels

A tunnel-boring machine
A tunnel-boring machine
Photo courtesy Department of Energy

Today, the New York City subway system is in the middle of a massive renovation. One of the planned additions is a new tunnel stretching all the way from Long Island to Manhattan. A tunnel-boring machine (TBM) will do much of the digging.

A TBM is a machine so large that it usually has to be transported in pieces. It uses discs and scrapers to crush and remove rock and debris, creating a tunnel. A conveyor removes this debris from the tunnel so crews can dispose of it. Although it moves slowly, a TBM can dig through both hard bedrock and softer soil, and it supports the tunnel as it digs.


But machines like this didn't exist during the construction of the world's first subways. Building crews had to excavate the subway lines in cities like London and Paris by hand. This was slow, difficult, dangerous work. For example, digging the New York City subway tunnels required close to 8,000 laborers. Thousands sustained injuries during construction, and more than 60 died. Improved construction methods haven't completely prevented subway construction accidents. In January 2007, a collapse at a subway construction site in Sao Paulo, Brazil buried a minibus and several dump trucks and created a 260-foot-wide crater.

Through the years, crews have used a variety of methods to excavate the subway tunnels. Some have blasted rock with dynamite, and others have used movable shields to protect diggers while excavating hollow tubes under streets and buildings. In the 1950s, some crews started using the New Austrian Tunneling Method (NATM), a collection of techniques for determining how and where to dig. You can learn more about other modern tunneling methods in How Tunnels Work.

In a cut-and-cover excavation, crews dig a trench and cover it with a temporary or permanent road surface.
In a cut-and-cover excavation, crews dig a trench and cover it with a temporary or permanent road surface.

One of the most commonly-used methods of early subway construction was the cut-and-cover method. This method required workers to do exactly what the name suggests -- cut a deep trench and cover it up. In order to make a stable covering over the excavation site, workers drove piles on either side of the trench. Then, they placed trusses and beams across the trench, using the piles for support. A temporary or permanent roadway could rest on this surface. The beams and trusses could also hold hanging supports for the pipes and conduits unearthed during the tunneling process. With this method, crews could usually create a tunnel that was deep enough for a train to travel through but shallow enough to avoid hitting nearly impenetrable bedrock.

A finished subway tunnel
A finished subway tunnel
Photo © 2001-2007 Stock.Xchng

This method was safer and more practical than digging horizontally underground. However, crews generally used city streets as guidelines for where to dig, which caused the complete, but temporary, destruction of existing roads. Planners were willing to accept this inconvenience because using the roads as a guide made tunneling easier. First, it allowed planners to make sure that the subway went where people needed it to go. Second, it reduced the likelihood of encountering building foundations or otherwise damaging existing structures.

But following the road doesn't always prevent work crews from running into unexpected obstacles. We'll take a look at some of these in the next section.

Tunnel Obstacles

Subway entrances make dealing with buried city infrastructure inevitable.
Subway entrances make dealing with buried city infrastructure inevitable.
Photo © 2001-2007 Stock.Xchng

Beneath the streets in every city, subway crews can find a maze of water and sewer pipes, electrical conduit, cables and pneumatic tubes. Modern tunneling machines can allow workers to dig a tunnel below these obstacles, but at some point the tunnel has to reach the surface. Many subway systems include a series of shafts that act as emergency exits, and all have entrances that people can reach from the street level. This makes it impossible for crews to completely avoid the existing city infrastructure.

Sometimes, workers have to reroute existing pipes and cables before construction can continue. In other cases, workers can excavate around them and suspend them from the surfaces above. Occasionally, workers uncover pipes or conduits that don't appear on any city blueprint -- this adds the extra step of determining exactly what purpose the pipes serve and whether they can be removed. But in some cases, working around the pipes and tubes is the easy part. In cities around the world, crews have found a number of other natural and manmade obstacles when excavating subway tunnels. A common difficulty involves underground water. Workers can discover anything from loose, wet soil to aquifers, or sources of ground water, while digging. Sometimes, crews can use pumps or dig dewatering wells to remove the water. Some water sources require more extreme measures. During the excavation of the Paris subway tunnels, workers used tubes of low-temperature calcium chloride to freeze unmanageable mud, allowing them to remove it as though it were solid clay.


In addition to underground water, many subways have to cross rivers and other aboveground bodies of water. Sometimes, a crew can dig under a river using modern tunneling machines. But in some cases, the soil below the river is too wet and muddy to manage. Digging under a river can also be particularly dangerous -- during the excavation of the Paris subway lines, attempts to dig under the Seine led to several drownings. In another attempt, workers dropped sealable enclosures to the bottom of the river, then used compressed air to force all the water from inside the enclosure. Within the confines of the enclosure, crews could keep digging. The work was difficult, though, and these workers generally received higher wages than those who tunneled through ordinary soil and rock.

In the 1960s, workers in San Francisco used a version of the cut-and-cover method to create a tunnel through the San Francisco Bay. Workers dug a trench in which to bury prefabricated tunnel sections. Divers placed the sections in precise positions and secured them to one another. Flexible joints at each end help protect the tunnel from earthquakes.

In some cases, naturally-occurring geological formations can bring construction to a halt. In New York City, hard-to-cut stone called schist kept workers from digging very deeply into the ground below the city. Modern crews can use explosives or tunnel-boring machines to get through dense rock, but the earliest crews sometimes had to reroute subway tunnels to bypass impassable stone.

Workers excavate a site for a subway station in Prague.
Workers excavate a site for a subway station in Prague.
Public domain image

Finally, numerous crews have discovered manmade structures while digging subway tunnels, particularly in very old cities. Crews in Paris, for example, uncovered cannonballs, catacombs full of human bones and the foundations of historic buildings. The Paris subway also travels through very deep quarries that have existed since ancient Roman times. In some cases, the quarry floor was far below where the subway track needed to go. Workers had to build bridges inside the quarries. In other words, portions of the Paris subway are underground, elevated railways.

A network of tunnels is complex and challenging to construct, but it's only one small part of a subway system. We'll look at the other components in the next section.

Subway Systems

Photo © 2001-2007 Stock.Xchng

At first glance, a subway is simple -- it's a train that runs through a tunnel. Most of the time, the train consists of several connecting cars that contain durable seats as well as poles and straps for people to hold on to when the train is full. The train runs on rails, which often have the same gauge as other rail systems around the city. In New York City, for example, the subway tracks' gauge is 4 feet, 8.5 inches (1.4 meters), which is the same as major railroad tracks. This allows the subway to connect to other railways.

But a subway also requires several other systems that riders can't always see. In some subway systems, the trains themselves, known as rolling stock, are extremely complex. The subway in Copenhagen, Denmark, currently in construction, uses completely computerized, driverless trains. The ongoing upgrade to the New York City subway will replace its older cars with automated cars, each of which will contain multiple computer systems. These systems will control everything from the interior lights to navigation. The trains will even have the capability to use brake heat to generate power. In such automated systems, a collection of sensors detects trains' locations in relation to one another. Computer programs keep track of where all the trains are, reducing the possibility of human error and improving the overall efficiency of the system.


Usually, automated subways also include surveillance systems, such as closed-circuit TV, to allow people to monitor trains' progress and safety from a control room. Two-way radio systems allow passengers to talk to employees in the control room or summon help in case of an emergency. Sensors also detect objects near the train or in doorways, so people don't get caught between the doors or hurt themselves trying to board the train before it departs.

Commonly-used home signals, also known as interlocking signals
Commonly-used home signals, also known asinterlocking signals

Subway systems that don't use automated trains have an extensive collection of signs and signals to help drivers operate the trains safely. Signs mark everything from speed limits to locations of fire extinguishers. Signals typically use colored lights to let drivers know when to stop, whether the track ahead of them is occupied and when to proceed with caution. Infrared sensors, capacitance plates or short-circuits created by the cars' wheels can let a signal know when a train is present. That signal can communicate with adjacent signals, ensuring that two trains do not try to occupy the same section of track at the same time.

Some signals also use physical mechanisms to make sure drivers obey them. For example, some signals can physically activate an emergency brake on a subway train if the driver continues past a stop signal. In the original New York City subway tunnels, drivers had to use a key to reset stop signals before they could proceed. The term keying by, still used in some signal situations, comes from this procedure.

A few early subways used steam engines, but in most existing subways, the trains, tunnel lights and station equipment all run on electricity. Overhead wires or an electrified rail known as the third rail supplies power to the trains. The third rail lies outside or between the subway tracks, and a wheel, brush or sliding shoe carries the power from the rail to the train's electric motor. In the New York City subway system, the third rail carries 625 volts of electricity, and the original lines required their own power plant to operate. A series of cables and substations carried the electricity from the power plant to the third rail.

Electrical power also controls the subway's ventilation system. Many subway systems include numerous sections of above-ground track and station entrances that are open to the air. However, natural air circulation from these sources isn't enough to keep the air in the tunnels breathable. Subways have an extensive series of fans and air shafts that circulate fresh air. The amount of circulation required is immense -- the planned ventilation system to be included in the New York City subway upgrade will move 600,000 cubic feet of fresh air every minute.

All of these systems add up to a lot of moving parts, many of which are underground and relatively inaccessible. In the next section, we'll explore what it takes to maintain a subway and keep it running.


Running and Maintaining a Subway

The elements and sensors commonly found in a geometry train
The elements and sensors commonly found in ageometry train

Most subway trains run along rails that have been in place for years, sometimes since the subway opened. Weather and daily wear and tear take their toll on the tracks. The rails of the New York City subway, for instance, are made from 39-foot (11.8-meter) lengths of carbon steel. Each rail is 5.5 inches (13.9 centimeters) high and 2.5 inches (6.35 centimeters) wide. Trains weighing as much as 400 tons (362.8 metric tons) run along these rails 24 hours a day, every day. In addition, the record temperatures range from 24 degrees Fahrenheit (-4 degrees Celsius) in January to 102 degrees Fahrenheit (39 degrees Celsius) in July [Source: BBC Weather]. Sections of track exposed to the elements encounter rain, snow, sleet and other precipitation every year.

All of these factors can affect the rails' surface and alignment. If the rails deteriorate or shift, the trains could derail as a result. For this reason, transit employees have to constantly monitor the state of the rails. To do this, they use a geometry train.


Railway and subway systems around the world use some type of geometry train to keep an eye on the tracks. These are cars that travel along the tracks, using lasers mounted to the front and underside to take precise measurements of the rails. In New York, the geometry train runs nonstop. Employees ride inside, analyzing the measurements and ordering repairs for any section of track that is more than 1.25 inches (3.1 centimeters) out of alignment.

The geometry train can also help employees prevent fires within the subway tunnels. Litter or other debris near the subway tracks can catch fire, quickly filling a tunnel with smoke. To prevent this, employees use infrared sensors to pinpoint hotspots near the rails. They use fire extinguishers to remove any threat of fire.

The employees who monitor the rails from the geometry train are only a few of the people required to keep a subway system running. Virtually every subway system also employs custodial, security and emergency medical staff. Systems with automated trains employ supervisors to work from the control room, and systems with manually operated trains employ both operators and control-room staff. In addition, subways have a management staff to create and implement a budget for running the subway and to coordinate renovations and expansions.

Many subways are not financially self-sufficient and must rely on government support to stay in business. In New York, for example, only about half of the money required to run the subway comes from riders' fares. For this reason, keeping a subway running involves careful interaction between government agencies and approval processes for planned upgrades.

We'll take a look at expansions and threats to subway systems in the next section.

Expansions, Upgrades and Threats

A November 2004 collision at Woodley Park-Zoo/Adams Morgan Station in Washington, D.C.
A November 2004 collision at Woodley Park-Zoo/Adams Morgan Station in Washington, D.C.
Public domain image

Since subways are intended to be a permanent part of a city's infrastructure, both expansions and renovations are inevitable. Cultural changes and major events can also have a profound impact on when and how a city upgrades and maintains its subway. For example, emphasis on energy conservation has led to increased use of the subways and the need for expansion in some major cities. Other events have had the opposite effect -- at the end of World War II, many people in cities around the world purchased cars and stopped taking the subway. Some systems fell into disuse, and local governments spent less and less money maintaining them. Before long, some systems, like the New York City system, fell into disrepair.

For such subways, bringing the trains, stations and tunnels back up to par can be expensive. The Metropolitan Transit Authority approved a $17.2 billion bid to improve the New York City subway in 1994. Even for well-maintained systems, authorities frequently must assess the need for extensions to the line or upgrades to equipment and rolling stock. In many cases, such upgrades are necessary to ensure the safety of employees and riders.


But even with a round-the-clock staff and top-of-the-line improvements, a few factors threaten subways. Some of the most common dangers are:

  • Fires ignited when sparks from the wheels or the third rail ignites litter or debris
  • Floods during extreme weather or when drainage systems fail
  • Crashes caused by pilot or signal error
  • Derailments caused by damaged tracks or foreign objects

In addition, insufficient security measures in some subway systems have led to graffiti, theft, assault and other crimes. Some systems have even experienced terrorist attacks. A sarin gas attack in the Tokyo subway in 1995 killed 12 people and hospitalized 493 others. Terrorists detonated bombs in three London subway trains during rush hour on July 7, 2005. After the September 11, 2001 terrorist attack on the World Trade Center, the collapsing towers destroyed a subway station and damaged portions of the track under the buildings.

Subways, along with the signs and warnings associated with them, have become part of popular culture in many cities.
Subways, along with the signs and warnings associated with them, have become part of popular culture in many cities.
Photo © 2001-2007 Stock.Xchng

On the other hand, subway systems around the world have often provided safety and shelter. During World Wars I and II, residents of London and Paris sheltered in the subway tunnels during air raids. Members of the French Resistance in World War II also used the tunnels to communicate with one another and to travel through the city. In addition, homeless people take refuge in many subway stations throughout the world. In New York City, this has led to the belief that there are organized societies of people known as "mole people" living underground. However, although researchers agree that people do live in the tunnels, there is little conclusive evidence of the societies described in some unverified accounts.

To learn more about tunnels, subways and related topics, check out the links on the next page.

Lots More Information

Related HowStuffWorks Articles

More Great Links

  • Atlas Dewatering. "Market Sectors: Subway Construction." (2/23/2007)
  • Britt, Robert Roy. "Subway Flooding: A Hidden and Neglected Risk." LiveScience. 1/14/2005 (2/23/2007) 050114_underground_floods.html
  • Cudahy, Brian J. "Under the Sidewalks of New York: Revised Edition." Viking Penguin. 1979.
  • Discovery Channel Online. Subways in America. subways/interactive/interactive.html
  • Discovery: Extreme Engineering: American Subways. Episode 103, 2003.
  • Dutemple, Lesley A. "The New York Subways." Lerner Publications Company. 2003
  • Feinman, Mark S. "Early Rapid Transit in Brooklyn." (2/23/2007)
  • Hovey, Tamara. "Paris Underground." Orchard Books. 1991.
  • McNeese, Tim. "The New York Subway System." Lucent Books. 1997.
  • Metro Bits: World Metro List (2/23/2007)
  • Middleton, William D. "Metropolitan Railways: Rapid Transit in America." Indiana University Press. 2003.
  • New York City Subway: Its Construction and Equipment (2/23/2007)
  • New York City Transit. Subways. (2/23/2007)
  • Orestad Development Corporation. "Tunnel Construction." (2/23/2007)
  • Railway Technical Web Pages: Electric Traction Drives
  • Subway Signals: A Complete Guide (2/23/2007)
  • The Associated Press. "Brazilian Subway Construction Site Turns into Crater, Swallowing Three Trucks." (2/23/2007) america/LA-GEN-Brazil-Subway-Crater.php
  • Transit Toronto. "How Cut and Cover Worked in 1951." (2/23/2007)
  • Washington Metropolitan Area Transit Authority (2/23/2007)