High-speed trains exist in various countries throughout the world, surpassing speeds of 200 mph (321 kp/h). However, many often wonder what powers these trains to reach such high speeds.
So, how are high speed trains powered? Electricity. High speed trains receive their electric power from over head wires, mostly at a voltage of 25 kV 50 Hz, and is collected via a pantograph atop the train.
Wires strung along a set of catenary are the most common types of powering high speed trains, as it is commonly the most energy efficient way, as it reduces the reliance on fossil fuels, and implements a new, convenient way of travel.
Overhead catenary is an effective and energy efficient way to operate high speed trains. Overhead wires or catenary are fed electricity through feeder stations along the railway, which have access to high capacity electrical grids.
There are various elements of a catenary wire. Oftentimes, two wires are strung above the tracks, the first wire, called the messenger wire, which supports the contact wire that makes contact with the pantograph. The function of the messenger wire is to keep the contact wire straight above the tracks for maximum current collection, especially at high speeds. The two wires are held together via a drop wire, which attaches to the two wires every few feet, thus, increasing its tension. By this connection, the contact wire and messenger wire are connected electrically.
High speed trains operated by overhead wires are very energy efficient, as once the electricity travels through the train, the unused current is then filtered back into the overhead lines for the next train to utilize.
The type of wires utilized for high speed operation must be able to handle the high heat and friction generated from a passing pantograph. Thus, oftentimes, the contact wire is constructed of copper to further strengthen the connectivity between the wire and the pantograph.
The messenger wire is oftentimes constructed of multiple metals, such as copper, aluminum, and steel, and must be equally as strong as the contact wire and must have strong conductive properties. In most cases, the messenger wire would be constructed by nearly twenty smaller wires inside a cable.
Tensioning is imperative in high speed rail operation, as it is important to prevent wire damage and other issues related to standing waves. Mechanical tensioning is carried out utilizing hydraulics or weights. This method, known as auto tensioning, is designed to keep the tension of each wire between 2,000 and 4,500 lbf to prevent any slack or rapid movements in the wire.
For ease of maintenance, sections of catenary along a high speed railway are separated into different sections. Where two sections meet is called a section break, where an insulator ensures the pantograph has constant contact with the contact wire. Oftentimes, to ensure pantograph contact, two contact wires and four drop wires are utilized.
Additional breaks include a neutral break, which take place when multiple power grids are supplying power to the overhead line. A phase break is a section of un-electrified catenary where two grids meet with different voltages from separate power grids. In between the phase break, an insulator is implemented to ensure constant pantograph contact with the catenary.
This is mainly only utilized in an AC system, as AC current is constantly cycling in polarity, and it is difficult to determine whether each electrical current is sufficiently synchronized with each other. Phase break is meant to prevent the pantograph from creating a spike in current, which could cause a power line disabled for maintenance to attract current.
Pantographs and Traction Motors
High speed trains collect power from the overhead alternating current (AC), wires, which transfers the energy to the transformer. The transformer then transfers the energy to the axle brushes, which the energy is then transferred to the primary rectifier, where the energy is converted into direct current (DC), which then travels to the primary inverter, where it is turned into 3 phase AC current, which is then fed into the traction motors thus, turning the wheels.
It is imperative that the carbon insert on surface of the pantograph experiences even wear. When a train is traveling on a straight section of track, the pantograph sways left and right, creating even wear on the copper insert. On a curved section of track, the pantograph crosses completely over the contact wire, once again creating even wear.
High speed trains are equipped with a special type of pantograph, called the “half pantograph” which due to its shape, resembles the letter “Z”. Also called the Faiveley pantograph, after its curator, Louis Faiveley, which was designed for various uses, however, most notably for high speed operation.
Contrary to popular belief, the Faiveley pantograph can be operated in either direction without affecting the efficiency of the equipment. This type of pantograph is utilized on most high speed networks such as the TGV, Shinkansen, and German ICE trains. Additionally, it is oftentimes utilized on high performance locomotives such as the Siemens Vectron, and Taurus locomotives.
Pantographs are lowered and raised via air pressure, and are equipped with a fail safe mechanism to prevent damage to the pantograph. The fail-safe mechanism lowers the pantograph in the event that the carbon insert on the top becomes damaged or becomes dislodged completely. Upon any of these circumstances, the air is released, and the pantograph drops down.
Alternatives to Electric Power
In the early days of high speed rail travel, various gas turbine and diesel electric powered trains were proposed. The prototype TGV train set, named “TGV 001”, was a gas turbine powered train set, which was powered by helicopter turbines. The train set the world speed record for non-electric traction in December 8, 1972, when it reached 198 mph (318 kp/h), a record that has yet to be broken. However, due to the oil crisis in the seventies, it was deemed not feasible to utilized gas turbine powered train sets, thus, electric traction was implemented.
The United Kingdom tried their hand at gas-turbine powered high speed train sets as well, with the introduction of the APT-e in 1972. Built by British Rail’s Derby Works, the APT-e is best known for its record breaking run on the Great Western Mainline, on BR’s Western Division, where it reached a speed of 152.3 mph (245.1 kp/h) between Swindon and Reading.
Although the experiment was successful, the set was never meant to enter revenue service, and was designed to test the high speed capabilities of Britain’s rail network. Although an experimental unit, the APT-e paved the way for the highly successful HST, which has been a staple on Britain’s rail network since its implementation in 1976.
Germany’s ICE train introduced diesel multiple units (DMU) in 2001 to replace locomotive hauled trains on DB’s existing rail lines. Designated the ICE TD, the sets were produced in a joint venture between Bombardier and Siemens, and operated on the route between Berlin and Copenhagen. However, the units suffered from astronomical maintenance costs, leading DB to not pursue a mid-life rebuild, thus, all were retired in 2017.
The most successful non-electrified high speed train set was British Rail’s High Speed Train (HST), which entered service in 1976, and has become a staple on nearly every railway in the country. Development of the HST began when Britain sought to implement high speed rail to its network, however, electrifying various rail lines throughout the country was not economically feasible during the seventies. Thus, diesel-electric traction was decided upon, and design was derived from the highly successful APT-e experiment.
The HST revolutionized various routes throughout the network, with its 125 mph (201 kp/h) top speed. The HST replaced various services operated by Class 55 Deltics on the eastern region, and Class 50s and 47s on the Western Region. With its abundant success, the HST continues to serve the United Kingdom, however, they are slowly being phased out on many routes by new Hitachi IET train sets.
Non fossil fuel alternatives have been proposed as well, such as the Magnetic levitation trains (Maglev), which have garnered much interest in recent history since its implementation on a test route in Germany in 1984. However, only the Shanghai Maglev has been operational in revenue service, as the German route was a test bed, and is due to be disassembled.
The Maglev operates via a series of linear arrays of electromagnetic coils, one being located on the guide-way, and the other attached to the train, which allows the train to hover over the guide-way. Although the Maglev concept is intriguing, construction and maintenance costs have deterred many from utilizing the technology. Instead, conventional rail high speed trains have been preferred.