Umair Gaines
The subway is a train that runs through a tunnel, and most subways run on electricity. The New York City subway, for example, is primarily electric and is one of the largest and busiest subway systems in the world. The electrical supply for the New York subway is designed so that in the event of an accident, trains can be operated on other tracks with separate and independent channels of electrical supply. The third rail provides electrical power to the subway cars, and the return current usually flows through one or both running rails. Some subways, such as the Paris Metro, use a live rail to feed the current, while others, like the London Underground, use a third rail for current feed and a fourth rail for current return.
| Characteristics | Values |
|---|---|
| Power source | Electric |
| Power requirement during peak hours | 495,900 kilowatts |
| Annual power usage | 1.8 billion kilowatt hours |
| Power source for trains, tunnel lights, and station equipment | Electricity |
| Power source for ventilation systems | Electricity |
| Gauge of tracks in New York City | 4 feet, 8.5 inches (1.4 meters) |
| Type of rail used | Third rail |
| Voltage of third rail | 600 volts DC |
| Type of contact shoe | Horizontal |
| Location of contact shoe | Below, above, or beside the third rail |
| Type of rail used in New York Central Railroad, Philadelphia's Market-Frankford Line, and Hamburg's Hochbahn | Bottom contact rail (Wilgus-Sprague system) |
| Type of rail used in Manchester-Bury Line of the Lancashire & Yorkshire Railway | Side contact rail |
| Voltage of third rail in Hamburg S-Bahn | 1200 V DC |
| Type of rail used in Copenhagen Metro, Taipei Metro, and Wuhan Metro | Third rail |
| Type of rail used in Sapporo Subway | Bottom-powered railways |
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What You'll Learn
Electric traction powers trains
Electric traction can be supplied through rechargeable energy storage systems, such as batteries, or stationary sources like a third rail or overhead wire. The third rail, located close to the ground, provides power to the power-train and ancillaries of subway cars. The return current usually flows through one or both running rails, which are connected to the substation. The third rail can be designed with top, side, or bottom contact, with bottom contact being the most effective as it covers most of the rail and protects it from cold weather.
The first electric locomotive was created by Robert Davidson in 1839, and ran on the Edinburgh-Glasgow railway at 4 miles per hour. Thomas Edison also built a small electrical railway in 1880, using a dynamo as the motor and the rails as the current-carrying medium. The first implementation of industrial frequency single-phase AC supply for locomotives was in 1901, and the first main-line three-phase locomotives were supplied to the Burgdorf-Thun railway in Switzerland in 1899.
Today, electric locomotives use brushless three-phase AC induction motors powered by inverters. Electric traction has become common in many countries, with high-speed rail systems such as the Shinkansen and TGV using electric trains.
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Third rail systems
A third rail system is a method of railway electrification that provides power to trains through a conductor rail placed alongside or between the running rails. This additional rail typically carries 600 to 750 volts of direct current (DC) electricity, although there are variations, such as the Bay Area Rapid Transit (BART) system in San Francisco, which uses 1000VDC. The third rail system is commonly used in underground train systems, such as the Boston MBTA and the Metro in Washington, D.C. It is also used in some overground train systems, although this is less common in the United States.
The third rail system offers several advantages. One key benefit is its environmental friendliness, as it is purely electric and does not rely on fossil fuels, resulting in no exhaust emissions. This makes it a cleaner and more sustainable option compared to steam or diesel-powered trains. Additionally, the third rail system is ideal for tunnels and underground environments due to its low clearance height requirements. Unlike overhead wiring systems, it does not need additional structures to carry wires, making it suitable for tunnels with low clearance heights. Furthermore, the third rail system is generally more cost-effective than overhead wiring systems, as it requires fewer infrastructure modifications and is less susceptible to weather-related disruptions caused by strong winds, heavy snowfall, or ice accumulation.
However, the third rail system also has some limitations. One of the main concerns is safety, as the high-voltage electricity it carries poses a significant hazard to anyone on the tracks. While safety measures such as platform screen doors, strategic placement of the conductor rail, and insulating cover boards are implemented to mitigate these risks, the system has a reputation for being dangerous. Additionally, the mechanical impact on the contact shoe and the gaps in the conductor rail at level crossings and other intersections can restrict the top speed of trains, making the third rail system less suitable for high-speed railway applications.
The history of the third rail system dates back more than 100 years, with early implementations in the New York Central Railroad near Grand Central Terminal (1907), Philadelphia's Market-Frankford Line (1907), and the Hochbahn in Hamburg (1912). Over time, the technology evolved, and by the 1920s and 1930s, it was adopted by systems such as the Berlin U-Bahn, Berlin S-Bahn, and Moscow Metro. Today, the third rail system continues to be used in various urban transit systems worldwide, including the Copenhagen Metro, Taipei Metro, and Wuhan Metro.
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Overhead wiring
One advantage of overhead wiring is that it allows trains to achieve higher speeds compared to third-rail systems. This is due to mechanical limitations on the contact with the third rail. Overhead wiring is also preferred in cities because it does not require very high speeds and causes less visual pollution.
However, overhead wiring has some disadvantages. It is susceptible to strong winds, freezing rain, and thunderstorms, which can bring down the wires and disable trains. Additionally, lightning strikes can cause power surges or breaks in the wires, disrupting the power supply.
Some countries, such as Japan, South Korea, and Spain, are more inclined to adopt overhead wiring for their urban railways.
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Bottom-powered railways
The third rail provides electrical power to subway cars. Bottom-powered railways, also known as bottom contact rails or the Wilgus-Sprague system, were pioneered by the New York Central Railroad on the approach to New York's Grand Central Terminal in 1907. The Philadelphia Market–Frankford Line and the Hamburg Hochbahn also implemented this system in 1907 and 1912, respectively.
The third rail is located close to the ground and carries a high current to transfer power to the train, resulting in high resistive losses. The electrified rail poses a hazard to anyone on the tracks, and various measures are taken to mitigate this risk, such as platform screen doors or placing the conductor rail away from the platform. The conductor rail can also be covered to protect track workers and the rail itself from environmental factors like frost, ice, snow, and leaf-fall.
The New York City subway, one of the largest and busiest systems in the world, is primarily electric, relying on electric traction to power its trains. The ventilation system in the New York subway, which includes fans and air shafts to circulate fresh air, is also electrically controlled.
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Electrical supply safety
The third rail, also known as a live rail, electric rail or conductor rail, is a method of providing electric power to a subway train. The third rail is placed alongside or between the rails of a railway track. The third rail is energised at 600 volts DC and powers the train, subway cars and ancillaries.
The third rail presents an electric shock hazard, especially with high voltages (above 1500 V). To reduce the risk of electric shock, platform screen doors can be used, or the conductor rail can be placed on the side of the track away from the platform. The conductor rail can also be covered with a coverboard.
To further ensure electrical supply safety, the third rail is electrically separated from the contact rails and positive feeders. This permits the use of direct-current circuit breakers, which can be set to open automatically at low currents, reducing the intensity of arcs that may occur in the subway in the event of a short circuit.
In addition, the New York City subway has separate and independent channels of electrical supply for each track. This ensures that in the event of an accident or derailment, trains can continue to operate on the unaffected tracks with their own electrical supply.
Modern tram systems have also implemented a segmented ground-level power supply, where each segment is electrified only when covered by a vehicle using its power, reducing the risk of electrical injury.
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Frequently asked questions
Yes, the New York City subways are primarily electric. The New York City subway is one of the largest and busiest in the world and it relies on electric traction to power its trains.
The New York City subway uses a third rail to power the trains. The third rail provides electrical power to the power train and ancillaries of the subway cars. The third rail is energised at 600 volts DC.
Some countries, like Japan, South Korea and Spain, use overhead wiring for their urban railways. Another method is to use a fourth rail to carry the return current, which is used by a few steel-wheel systems.
