A tunnel-boring machine (TBM) is used to do much of the digging. It 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.
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.
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.
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.
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.
WATERPROOFING
After digging subway tunnels, workers use a variety of materials to make them waterproof. Common historical methods included lining tunnel floors with gravel, applying waterproof material coated in concrete and lining the tunnel with asphalt and brick. Modern subway crews often use materials like Shotcrete, or sprayable concrete, to finish tunnel walls. Many tunnels, particularly those that house subway stations, also have a decorative layer of tile or stone.
Subway Systems
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.
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
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. 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.
Expansions, Upgrades and Threats
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.
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.
