Molly Ayala
High-speed trains, or bullet trains, are generally defined as those with operational speeds above 250 km/h (155 mph). Only 16 nations have high-speed railways, with China boasting the world's longest network at 27,000 km of tracks. While some high-speed trains are diesel-powered, most are electric and are powered by overhead lines. This article will explore the various methods by which bullet trains are powered by electricity.
| Characteristics | Values |
|---|---|
| Power source | Electricity, diesel, gas turbine |
| Speed | Above 250 km/h |
| Examples | Shinkansen, TGV, German ICE trains, Siemens Vectron, Taurus locomotives, APT-e, HST |
| Location | China, Japan, Spain, France, Germany, United Kingdom, California |
| Track type | Conventional, Maglev |
| Energy source | Solar, fossil fuels, overhead catenary |
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Overhead power lines
The overhead catenary system is an effective and energy-efficient way to operate high-speed trains. It consists of two wires: the messenger wire, which supports the contact wire that makes contact with the pantograph, and the contact wire, which maximises current collection at high speeds. The wires are held together by a drop wire, which increases tension by attaching to both wires at regular intervals.
The high-speed trains then collect power from the overhead alternating current (AC) wires. The energy is transferred to a transformer, which then passes it on to the axle brushes. From there, the energy is transferred to the primary rectifier, where it is converted into direct current (DC). The DC current then travels to the primary inverter, where it is transformed into three-phase AC current, which powers the traction motors and turns the wheels.
While most high-speed trains use overhead power lines, there are exceptions. For instance, the Eurostar, which links Paris and London, used third-rail power supply when it first entered the latter city before a new line could be opened. However, it could not run at its maximum speed using this power source. Similarly, the British HST and some other high-speed trains with speeds up to 200 km/h are diesel-powered.
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Diesel engines
While most bullet trains are powered by electricity from overhead lines, some are powered by diesel engines. The UK HST, for example, is diesel-powered, as some parts of the UK are not electrified.
Diesel–electric locomotives have power outputs that are independent of road speed, as long as the unit's generator current and voltage limits are not exceeded. The unit's ability to develop tractive effort (or drawbar pull/tractive force) varies inversely with speed within these limits. The prime mover's power output is determined by its rotational speed (RPM) and fuel rate, which are regulated by a governor or similar mechanism. The governor is designed to react to the throttle setting and the speed at which the prime mover is running. Maintaining acceptable operating parameters was a key design consideration in early diesel–electric locomotive development, leading to the complex control systems in place on modern units.
Diesel–mechanical propulsion is limited by the difficulty of building a reasonably-sized transmission capable of handling the power and torque required to move a heavy train. There have been a few successful attempts to use diesel–mechanical propulsion in high-power applications, such as the DSB Class MF.
Diesel–hydraulic locomotives use one or more torque converters, in combination with fixed-ratio gears. Drive shafts and gears form the final drive to convey the power from the torque converters to the wheels, and to effect reverse. The difference between hydraulic and mechanical systems is where the speed and torque are adjusted. In the mechanical transmission system, if there is a hydraulic section, it is only to allow the engine to run when the train is too slow or has stopped.
Some locomotives can operate as either electric or diesel locomotives. For example, the Long Island Rail Road, Metro-North Railroad, and New Jersey Transit Rail Operations use dual-mode diesel–electric/third-rail locomotives in non-electrified areas and New York City, due to a local law banning diesel-powered locomotives in Manhattan tunnels.
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Maglev trains
There are two primary types of maglev trains in operation: Electromagnetic Suspension (EMS) and Electrodynamic Suspension (EDS). EMS employs attractive magnetic forces between the train and the guideway to achieve levitation. Magnets are present on the sides and underside of the train, as well as on the guideway, allowing the train to hover about 1.3 cm (0.5 inches) above the track. A variation of EMS, known as Transrapid, utilizes an electromagnet to lift the train off the guideway, with magnets on the train's underside wrapping around the iron rails to maintain stability.
On the other hand, EDS systems utilize magnetic repulsion rather than attraction. These magnets are supercooled and superconducting, capable of conducting electricity for a brief period even after power loss. EDS trains are slower to lift off, requiring wheels to be deployed below approximately 100 km/h (62 mph). Once levitated, propulsion is provided by the guideway coils, which constantly change polarity due to alternating electrical currents.
The guideway, or track, plays a crucial role in the operation of maglev trains. It contains simple metallic loops made of conductive materials such as aluminum. When a magnetic field moves past these loops, it induces an electric current that generates another magnetic field. Three types of loops are strategically placed within the guideway: the first enables the train to hover, the second maintains horizontal stability, and the third provides propulsion through a combination of magnetic attraction and repulsion to move the train forward.
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Solar power
California is building a $100 billion bullet train that will be fully powered by solar energy. The project, which is set to be completed by 2030, will be the world's first high-speed train to run entirely on solar power. The train will connect many cities within California, with the ultimate goal of expanding across the US and into Vancouver, Canada. San Diego, Los Angeles, and San Francisco are some of the cities planned as part of this development.
The California High-Speed Rail Authority is preparing to begin discussions with potential suppliers of a $200 million utility-scale solar power system. This system will include 552 acres of solar panels, generating 44 megawatts of electricity—enough to power a city of 33,000 people. The solar panels will also support onboard electric batteries capable of storing up to 62 megawatt-hours of power. This stored energy will be crucial in maintaining the train's function in the Californian climate and ensuring the train can be self-powering if energy supplies fail.
The solar-powered train is expected to reach top speeds of 354 km/h (220 miles per hour) as it travels through the 171-mile Central Valley segment of the railway. The choice to route the train through Central Valley, one of the less affluent areas of California, was made with the goal of fostering economic growth in the region.
The construction of the solar-powered bullet train in California is a significant step towards renewable energy and global net-zero targets. The success of this project could be instrumental in encouraging similar initiatives worldwide.
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Alternating current
High-speed trains, including bullet trains, are predominantly powered by electricity, with some exceptions being diesel-powered. The most common method of transferring this electrical power to the trains is through overhead lines, also known as catenary wires. These wires are fed electricity through feeder stations with access to high-capacity electrical grids.
The electricity is then collected by a device called a pantograph, which is mounted on the roof of the train. The pantograph can be lowered and raised via air pressure and has a fail-safe mechanism that lowers it if the carbon insert on its surface becomes damaged or dislodged. This prevents damage to the pantograph and ensures even wear on the carbon insert.
The alternating current (AC) collected by the pantograph is transferred to a transformer, which then passes the energy to the axle brushes. From here, the energy is transferred to the primary rectifier, where it is converted into direct current (DC). The DC current then travels to the primary inverter, where it is turned back into three-phase AC current and fed into the traction motors, turning the wheels and propelling the train forward.
This process of converting AC to DC and then back to AC helps to ensure that the electrical current is sufficiently synchronized, as AC current constantly cycles in polarity. This synchronization is crucial for the stable and efficient operation of the high-speed train.
While most high-speed trains use overhead catenary wires, some newer systems, like the maglev trains, employ different methods. Maglev trains, which use magnets to levitate above the tracks, can also receive power through coils along the track, similar to wireless charging. Additionally, some trains, like the Eurostar, have used third-rail power supply in certain sections of their routes. However, this method has limitations in voltage and equipment durability due to the need to stop and start around switches and platforms.
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Frequently asked questions
Bullet trains are powered by electricity, which they collect from overhead wires or catenary. The electricity is transferred to a transformer, then to the axle brushes, then to the primary rectifier, where it is converted into direct current (DC). The current is then fed into the traction motors, turning the wheels.
The Tokaido Shinkansen in Japan, the TGV in France, and the German ICE trains are all examples of bullet trains.
Maglev trains, which use magnetic levitation, are capable of travelling faster than bullet trains with less environmental impact. However, they are very expensive to build. Gas turbine-powered trains have also been proposed, but the oil crisis in the 1970s made electric traction more appealing.
