Wiktor Wise
Rail electrification is the process of enabling trains to run on railway tracks using electricity, removing the need for engines powered by diesel or coal. Electric trains are more efficient than diesel-powered trains, which transfer about 30-35% of the energy generated by combustion to the wheels, while supplying electricity directly from an overhead power line transfers about 95% of the energy to the wheels. Electric trains are also more environmentally friendly, as electricity can be generated from diverse sources, including renewable energy. Electric locomotives are usually quieter, more powerful, and more responsive and reliable than diesel trains. They also have no local emissions, which is an important advantage in tunnels and urban areas.
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What You'll Learn
Power sources for electric trains
Electric trains are powered by electricity from overhead lines, a third rail, or on-board energy storage such as a battery or a supercapacitor. The electricity is typically generated in large and relatively efficient generating stations and transmitted to the railway network for distribution to the trains.
The first electric locomotive was built in 1837 by Robert Davidson of Aberdeen and was powered by galvanic cells (batteries). The first electric passenger train was presented by Werner von Siemens in Berlin in 1879. The locomotive was driven by a 2.2 kW, series-wound motor, and the train, consisting of the locomotive and three cars, reached a speed of 13 km/h. The electricity (150 V DC) was supplied through a third insulated rail between the tracks.
Today, most electric locomotives use AC motor-inverter drive systems that provide regenerative braking, allowing kinetic energy to be recovered during braking and put back on the line. The majority of modern electrification systems take AC energy from a power grid that is delivered to a locomotive and then transformed and rectified to a lower DC voltage for use by traction motors.
Overhead catenary systems, or "catenary", have a complex geometry and are usually designed by computer. The contact wire is held in tension horizontally and pulled laterally to negotiate curves in the track. The wire length is usually between 1000 and 1500 meters, depending on temperature ranges. The wire is zigzagged relative to the centre line of the track to even the wear on the train's pantograph as it runs underneath.
Third-rail systems can suffer disruption in cold weather due to ice forming on the conductor rail, and overhead cables are an attractive target for scrap metal thieves due to the high scrap value of copper.
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Overhead lines and cables
Overhead lines, also known as overhead wires, are electrical cables that transmit electrical energy to electric trains. They are situated over rail tracks, raised to a high electrical potential by connection to feeder stations at regular intervals along the track. The feeder stations are usually fed from a high-voltage electrical grid. Electric trains that collect their current from overhead lines use a device such as a pantograph, bow collector, or trolley pole. The pantograph is a device on the top of the train that makes contact with the overhead wire.
The overhead line structures and cabling can have a significant landscape impact compared to non-electrified or third rail electrified lines, which have minimal equipment above ground level. Overhead lines are considered by some to be "visual pollution" due to the many support structures and complicated system of wires and cables. This has led to a move towards replacing overhead power and communications lines with buried cables. Overhead lines can also be fragile and vulnerable to disruption due to mechanical faults or high winds, which can cause severe disruptions to train services.
The overhead catenary system, also known as the catenary, has a complex geometry that is typically designed by computer. The contact wire must be held in tension horizontally and pulled laterally to negotiate curves in the track. The wire length is usually between 1000 and 1500 meters long, depending on the temperature ranges. The wire is zigzagged relative to the centre line of the track to even the wear on the train's pantograph as it runs underneath. The tension of the wire is maintained by weights suspended at each end of its length.
Overhead lines are fed in sections, similar to third rail systems, but AC overhead sections are usually much longer. Each subsection is isolated from its neighbour by a section insulator in the overhead contact. The subsections can be joined through special high-speed section switches. In some cases, track magnets are used to automatically switch off the power on the train as it approaches a neutral section to reduce arcing.
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Third rail systems
The third rail system is ideal for use in tunnels (such as those in subway systems) with low clearance height as no additional structures are required to carry overhead wires. Third rails do not need to be erected under tension and are therefore not prone to breakage accidents. They are also highly reliable, with a low susceptibility to failures and outages. Since they are primarily used underground, they are generally not impacted by adverse weather and natural disasters.
The third rail system is generally associated with a low voltage (rarely above 750 V) and is far less used for main lines than overhead line systems, which permit a higher voltage and, therefore, more power. Because third-rail systems are located close to the ground, they present electric shock hazards, and high voltages (above 1500 V) are not considered safe. The risk of electric shock can be reduced by placing the conductor rail on the side of the track away from the platform or by covering the conductor rail with a coverboard.
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Electric traction systems
The power supply for electric railways must be safe, economical, and user-friendly. It is usually generated in large and relatively efficient generating stations and transmitted to the railway network, although some electric railways have their own dedicated generating stations and transmission lines. The power is then distributed to the trains through a nearly continuous conductor that runs along the track. This conductor can take the form of an overhead line or a third rail mounted at track level. Overhead lines are suspended from poles or towers along the track or from the ceiling of a tunnel and contacted by a pantograph. The third rail system uses a "shoe" to collect the current on the train.
Some electric traction systems provide regenerative braking, which turns the train's kinetic energy back into electricity and returns it to the supply system to be used by other trains or the general utility grid. This system is particularly advantageous in mountainous operations, as descending locomotives can produce a large portion of the power required for ascending trains.
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History of electric trains
The history of electric trains dates back to the mid-19th century, with early efforts focused on battery-powered locomotives. In 1835, attempts were made to propel railroad vehicles using batteries, and the first known electric locomotive was built by chemist Robert Davidson in 1837. This locomotive was powered by galvanic cells (batteries) and was later exhibited in 1841. However, the limited power of batteries prevented their widespread use.
The first successful application of electric traction came in 1879 when Werner von Siemens presented the first electric passenger train and locomotive powered by a generator in Berlin. This train reached a speed of 13 km/h and carried 90,000 passengers over four months on a 300-meter-long circular track. The electricity was supplied through a third insulated rail, and a contact roller was used to collect it.
The world's first electric tram line opened in Lichterfelde, near Berlin, Germany, in 1881, built by Werner von Siemens. This was followed by the opening of Volk's Electric Railway in Brighton, England, in 1883, and the Mödling and Hinterbrühl Tram near Vienna, Austria, in the same year. The latter was the first in the world to be powered by an overhead line.
The use of electric trains gained traction in the late 19th and early 20th centuries, particularly for underground and suburban lines. The first electrically worked underground line was the City and South London Railway, which opened in 1890 due to a prohibition on steam power. Similar motivations were seen in other cities, like New York, where smoke from steam locomotives was a nuisance in tunnels. Electric locomotives began operation on the New York Central Railroad in 1904, and the Pennsylvania Railroad electrified its entire territory east of Harrisburg, Pennsylvania, in the 1930s.
The advantages of electric trains, such as better energy efficiency, lower emissions, and lower operating costs, contributed to their increasing adoption. Electric locomotives are also quieter, more powerful, and more responsive and reliable than diesel engines. The flexibility in energy generation, including renewable sources, further favoured electrification.
Over time, electrification technologies evolved, and various systems were employed, such as overhead catenary systems and third-rail systems. The voltage and current used also varied, with historical examples including 3,000 V DC and 25 kV AC. Today, most electrification systems use AC energy from a power grid, which is then transformed and rectified to a lower DC voltage for use by traction motors.
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Frequently asked questions
Electric trains are substantially more energy-efficient, produce lower emissions, and are cheaper to operate and maintain than diesel trains. They are also usually quieter, more powerful, and more responsive and reliable.
Electrical cables can be used to transmit electrical energy to electric trains via overhead lines or a third rail. Overhead lines are electrical cables suspended from poles or towers along the track, while a third rail is a rail next to the track that has been electrified.
Overhead electrical cables can be visually unappealing and are vulnerable to severe disruption from mechanical faults or high winds. They are also attractive targets for scrap metal thieves due to the high scrap value of copper.
