Engineering better railways

By Philip Lechner   |   January 25, 2022

The pressure to decarbonise the economy affects every aspect of our lives, from what we eat to how we travel. It is driving innovation throughout the transport sector, and particularly in railways which, despite being one of the least polluting ways of travelling in terms of carbon dioxide equivalent (CO₂e) emissions per passenger mile, provide so much travel that their impact is significant.

For engineers, the complexity of the global rail market, and the wide array of technologies that enable it, means that there will be opportunities to innovate to achieve greater efficiency and lower emissions for decades to come.
 

The decarbonisation imperative

Why is decarbonising railways so important? According to the UK’s Office of Rail and Road, in 2019 – 2020, UK passenger trains consumed 4186 million kWh of electricity, up 5.3% on 2018 – 2019, and 476 million litres of diesel, up 1.5% on 2018 – 2019. This upward trend was matched by rising passenger numbers, which therefore drove emissions per passenger kilometre down to 35.1g, 4.9% less than in 2018 – 2019. Aggregate CO₂e emissions for electric and diesel passenger trains were 2400 ktonne, 2.7% down on 2018 – 2019.

What’s striking about these numbers is the impact that a 1% improvement in the efficiency with which energy is converted into passenger miles would have, saving almost 4.2 millionkWh of electricity, almost 5 million litres of diesel, and 2.4 ktonne of CO₂e emissions.

One of the main ways in which this sort of efficiency gain is going to be achieved, at least at the macro scale, is through electrification of the UK’s railways. In a 2020 plan for decarbonising train traction in the UK, Network Rail, which manages the UK’s rail infrastructure, said that of the 15,400km of UK rail which is as yet un-electrified, 11,700km should be electrified. Of the balance, 900km should be served by hydrogen-powered trains and 400km with battery-powered trains. The right technology for the remaining 2,400km of line is yet to be finalised, but the report suggested a further 1,340km of electrification, 400km of hydrogen-powered trains, and 400km of battery trains. A technology choice for the remaining 260km has yet to be made.

If this plan is implemented, up to 96% of passenger kilometres will be served with electric traction and 4% with hydrogen and battery units. For freight transport, around 90% of freight kilometres will be powered electrically, with the rest using diesel or other forms of traction.
 

Bridging the gap with batteries

Full electrification of the UK’s rail networks will take decades, during which train operators will have to work with partially electrified networks. One way of doing so will be to use battery-powered or battery-enhanced trains, an idea that has already attracted both start-ups and established train manufacturers.

Recent start-up Vivarail, based in Southam, Warwickshire, has developed a battery-powered train for use on metro, commuter, and regional services. It’s enabled by one of a set of technologies with which Vivarail says it can build new battery-powered trains, convert existing diesel trains, or add batteries to electric trains to extend their range. Vivarail has battery and hybrid trains in service, and one of the variants has a range of 60 miles on batteries alone. Vivarail has also built a charging system, consisting of a large battery bank trickle-charged by grid or green electricity and an under-train shoe-and-rail connection system. It claims this can recharge a train’s batteries in ten minutes.

Hitachi Rail is developing electric trains that can draw power from overhead wires for both traction and battery charging, and then switch to battery-only operation in areas where it is not possible or cost-effective to install the overhead power infrastructure. The company is also suggesting using batteries to replace some of the diesel power units on its current or future intercity trains, reducing fuel costs by up to 30% and enabling the trains to enter non-electrified stations in battery mode, creating a quieter, cleaner passenger experience.

Hitachi ABB Power Grids is supporting Hitachi Rail’s initiative by providing modular, containerised charging substations that can be distributed along popular routes to provide regular top-up charging. It is already using a similar approach for e-buses, for example on the route between Geneva airport and the suburbs. Here, the company has installed ‘flash’ charging stations at 13 of the route’s 50 bus stops. When an e-bus arrives at a charging stop, it connects to an overhead gantry and then charges for 20s at 600kW. CAF Power & Automation has developed and installed a similar system in Seville, Spain, fast-charging trams via an overhead pantograph that connects with a charging system while stationary at the terminus.

For rail, a similar system will take power from the national grid, convert it to 25kV and deliver it to a short section of overhead wires, through which the train can get a high-power flash charge for a few seconds.
 

The conversion challenge

As with a lot of e-Mobility solutions, the efficiency with which power is converted between forms in electric trains will be critical to reducing their effect on the planet. If you consider that the energy for an electric railway may be distributed from the power station at 400kV, and yet passengers expect to charge their phones from a 5V onboard USB port, there’s potential for many lossy conversion stages between the two voltage levels.

This concern is not unique to electric railways. DC-DC converter makers and the semiconductor companies that provide them with switching devices are constantly developing their circuitry and device architectures to achieve greater conversion efficiency. What makes this more difficult for companies serving the rail sector is the challenging environments in which they work, and the expectations of very long operating lifetimes.

Electronics for rail applications must survive environments that expose them to pollution and salt mist, wide temperature swings (–40 to +85ºC) and high humidity, as well as extreme shock and vibration. They are also expected to be resistant to fire and smoke and protected against interruptions, variations, and reversals of their supply voltage.

Many of these requirements are set out in strict standards, which can only be met through good engineering coupled with extensive electrical and environmental testing.

Applications for DC-DC converters for the rail industry tend to be split into trackside usages and on-train applications.

Trackside uses can include railway signalling control and enabling communications infrastructure. On-train applications may include traction, braking and lubrication system control; safety systems such as door control, fire control, signage, and CCTV; and passenger comfort features such as lighting, infotainment, and heating and ventilation systems.

On-board mount low voltage DC-DC converter modules are often potted, for protection against dust and moisture ingress, and may also have an embedded heatsink to conduct excess thermal energy away from the embedded converter circuitry to enable effective cooling. They’re supplied in standard formats that are widely used in the rail industry, such as what is known as a ‘half-, quarter- or eight-brick’ package.
 

Switching to silicon carbide

Most railway high voltage traction driver modules rely on silicon IGBTs, diodes and MOSFETs. But as efficient energy usage becomes increasingly important, some manufacturers are experimenting with using Silicon Carbide (SiC) devices instead. These devices can switch more quickly than silicon devices, which means that resonant parts of power-conversion circuits, such as coils, can be smaller. They also work at temperatures that would destroy a silicon device, allowing them to handle more power or use less cooling.

CAF Power & Automation, based in the Basque Country, is developing an electric traction power system using SiC devices, which it claims could provide energy savings of up to 15%, compared with conventional approaches. The train maker is working with the IKERLAN technology centre and Euskotren, a local public -transport operator, to develop and try out the technology.

CAF claims that using SiC will reduce losses in the traction converter alone by 70% and allow high switching speeds for greater efficiency. It will also support much higher operating temperatures than silicon and can dissipate that heat more quickly because it's a thermal conductivity that is three times higher than that of silicon. This helps simplify the cooling regime, and so contributes to reducing the volume and mass of the overall traction solution by 25%. This, in turn, makes the trains quieter, improving passenger comfort.

The work on this development has been underway since 2016 as part of a European Horizon 2020 research project, funded by the EU. The prior version of the traction system used a Si IGBT inverter and SiC diodes, demonstrating new technologies can be taken up incrementally and contribute significantly to the rail industry.

Innovation in the rail industry will be full of this sort of systemic optimisation opportunity, in which an improvement in one narrow aspect of a train’s design will lead to advantages in other aspects. The exciting challenge for engineers, as they work to reduce the carbon footprint of travel, will be in exploring their design options to achieve these kinds of systemic advantages.

To find out more about component solutions from our leading suppliers for the electrification of railways, visit our non-automotive transportation page. Alternatively, if you have a question or you would like to discuss your design in more detail,you can reach out to our team of field applications engineers in your local language.

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Philip Lechner

Philip Lechner studied electronics and telecommunications in Amsterdam before beginning his career i...

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