How Subways Work
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.
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.
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