Alongside the headline celebrations of 200 years of rail in Britain, 2025 also marks the 70th anniversary of the 1955 Modernisation Plan to update the rail network. Drawn up under Churchill’s second term after Labour had nationalised the railways in 1948, the plan called for a combination of mainline electrification as well as the introduction of diesel power to replace ageing steam locomotives.
Though its success is contested - the Beeching cuts that followed in the 1960s are often linked to its shortcomings - the legacy of the Modernisation Plan is undeniable. The mix of electrification and diesel laid out in 1955 is fundamentally what still powers Britain’s trains today. And while the sun may slowly be setting on diesel, only around 40 per cent of the UK network has been electrified over the intervening 70 years, presenting both a challenge and a unique opportunity for the rail sector.
“A pitifully small amount of the UK railway network is electrified,” Graeme Clark, Siemens Mobility’s head of Business Development for Rolling Stock, told The Engineer. “Electrification is the right way to go, but to electrify everything would cost a fortune. A lot of the route densities of traffic are not high enough in many cases to justify full electrification.”
The answer, according to Clark and many others, is battery trains, or battery electric multiple units (BEMUs). BEMUs are bi-mode trains that can be powered by overhead lines or third rails where available, operating in battery mode on parts of the network yet to be electrified.
For regional lines with no electrification – and little prospect of it in sight – batteries paired with fast-chargers could replace the diesel multiple units (DMUs) that have been the workhorses of UK rail for several decades. It’s a huge opportunity for operators and OEMs to modernise train fleets, improve passenger experience, and make a serious dent in the rail sector’s carbon emissions.
Siemens Mobility’s battery offering for the UK is the Desiro Verve, a BEMU evolution of its longstanding Desiro train family. Anyone who has travelled on Thameslink services will be familiar with the train, as more than 100 Desiro City variants are currently in operation on Thameslink’s main line routes in and around London. According to Clark, the average Verve passenger would be none the wiser they were travelling on a BEMU.
“The batteries are all outside the vehicle,” he said. “So if you see them from the outside, the only thing you would notice is it has got a few more boxes underneath it. There’s nothing inside the train at all, apart from seats and luggage racks and the things that the passengers want to see.”
The history of battery trains dates all the way back to the 1890s, with several successful iterations through the first half of the 20th century before diesel started to dominate. Early chemistries ranged from nickel-iron and nickel-zinc to the lead-acid batteries used by the British Rail BEMU that ran on Scotland’s Deeside Railway from 1958-1966.
Today, lithium-ion is the battery chemistry of choice for most mobility applications, though a variety of flavours are available. Trains are very different vehicles to cars, and what works for the automotive sector is not necessarily the right fit for rail. Finding the right balance between density, charging time and longevity is paramount, according to Clark
“We’re looking at lithium titanate oxide batteries,” he said. Though considerably lower in energy density than other types of lithium-ion chemistries, lithium titanate oxide (LTO) batteries are ideal for the rail sector. Trains are such hefty vehicles to begin with that additional weight is simply not as crucial as it is in sectors like automotive and electronics. And what LTO lacks in power density, it makes up for in charging speed and durability. According to Clark, Desiro Verve’s LTO batteries will require just a single overhaul throughout the life of the trains.
“Train life is typically 35 years,” he said. “And lithium titanate oxide batteries will last for half-life of the train at least. So we’ve only got to change or overhaul once, which makes it a very cost-effective solution.”
Lifecycle isn’t the only area where LTO’s durability shines. The chemistry is also inherently more stable than higher density Li-ion batteries like nickel manganese cobalt (NMC). “If you’ve looked on YouTube, you can actually hammer a nail into NMC batteries and they will explode or catch fire, and you can’t really put them out,” said Clark. “If you do that with a LTO battery, you can hammer a nail in it to really abuse it, and it won’t do anything nasty.
Electrification is the right way to go, but to electrify everything would cost a fortune. A lot of the route densities of traffic are not high enough in many cases to justify full electrification
Graeme Clark, Siemens Mobility
“That might sound a strange thing to do, but we have to face up to the fact that one of these trains, at some time in its life, is going to hit something on a level crossing, or a shopping trolley that somebody’s left at the side of the track or something. So we need to make sure it’s absolutely safe. And LTO is by far the best chemistry as well as being the longest life.”
Modelling by Siemens predicts that widespread adoption of the Desiro Verve could save 12 million tonnes of carbon emissions and £3.5 billion over 35 years, while supporting the government target of scrapping diesel-only trains by 2040. “We looked at all the networks in the UK…and if they’re likely to need new regional trains over the next 10 or 15 years,” said Clark.
What’s more, the £3.5bn figure is just a topline cost-of-ownership figure that doesn’t take into account value gained from the environmental benefits. “If you look at the government’s TAG guidelines, the Transport Advisory Guidelines on emissions, we would save probably another £3.5 billion over 35 years in reductions in CO2 as well,” said Clark.
Siemens Mobility is gearing up for production of the Verve at its Goole factory in East Yorkshire, with tenders ongoing with TransPennine Trains and Northern. However, UK operators have been somewhat slow to take the plunge on battery trains, understandably squeezing the most out of the DMU rolling stock that has served the network so well down the years. The change will come, but it will be gradual, with the 2040 target of scrapping diesel-only trains still some way off.
Across the Irish Sea, Iarnród Éireann (IE/Irish Rail) is moving slightly faster, adopting battery trains to build on its low base of electrification. The DART (Dublin Area Rapid Transit) is a commuter system that connects the Irish capital’s coastal suburbs with the city centre, its 53km of track currently the only part of the country’s main line network that’s electrified. Plans are in place to address that, with ambitions to electrify lines from Dublin to Ireland’s other major cities by 2050. But the same economic constraints exist in Ireland as in the UK.
“The amount of time that Irish Rail would have to invest in upgrading the infrastructure to extend the network is significant,” James O’Sullivan, product manager at Alstom, told The Engineer. “It would take a long time to see a return on that benefit.”
In the meantime, the DART+ project is aiming to triple the size of the commuter network around the capital to more than 150km. That expansion will see some main line electrification to the west of Dublin, served by EMUs. However, extended services north of the city to Drogheda will initially be enabled by bi-mode BEMUs. The Alstom X’Trapolis trains will travel as far as Malahide in north county Dublin on the existing overhead equipment, before lowering their pantographs and switching to battery power for the remaining 40km to Drogheda. Approval was recently granted to electrify the section of track, though delivery will be several years away.
“What they want to do is electrify all these sections fully,” said O’Sullivan. “But the battery trains give them instant decarbonisation…and they effectively create space for themselves to go and do this electrification without the pressure of looming decarbonisation deadlines.”
The first X’Trapolis trains arrived from Poland via Rotterdam in November 2024. Ireland’s slightly obscure 1600mm broad gauge – shared only with some parts of Australia and Brazil - means that factory track testing isn’t possible. This translates to a lengthy commissioning process on delivery, with overnight dynamic testing once passenger services finish. Eagle-eyed Dubliners have been teased with glimpses of the shiny new trains running empty through the night, their much-anticipated debut not scheduled until early 2026. A total of 185 Alstom X’Trapolis EMUs/BEMUs have been ordered to date, with as many as 750 electric carriages set to be delivered over a ten-year period.
Once the first tranche of carriages has been tested in battery mode and on overhead lines, a new fast-charging system in Drogheda will be put through its paces, ensuring the various interfaces are all operating as designed. Developed and installed by Alstom, the facility will mimic the DART’s 1500 V overhead equipment, rapidly powering the rooftop batteries for the 40km trek back across the non-electrified wilderness to Malahide.
“It’s a smart system, so it energises itself when the trains are arriving automatically,” said O’Sullivan. “The system is fitted with its own lineside batteries to draw on the grid and slowly trickle charge, and then rapid charge the trains when they need to minimise the turnaround time at the station.”
Short turnaround times are vital for both passengers and operators, and rapid charging will no doubt be integral to unlocking the potential of battery trains. Back in the UK, Great Western Railway (GWR) has been developing its own charging solution, testing it in West London as part of its early BEMU forays. Developed by Vivarail to work alongside the company’s retrofitted London Underground rolling stock, the Fast Charge system uses containerised trackside power banks to rapidly energise trains from below when they arrive into stations.
“We’re hooked into the 415 V, three phase domestic (supply),” Simon Green, engineering director at GWR, told The Engineer. “The lineside batteries obviously store it, so we can charge overnight and then we can use the electricity in big peaks to charge the train when it sits in the station.”
The trial at West Ealing was originally planned in collaboration with Vivarail, but when it entered administration in 2022, GWR and Network Rail stepped in and acquired the bulk of its assets. The Fast Charge IP moved across, along with several members of the Vivarail team.
“They were just starting to do fast charging testing in their Bletchley depot, starting to test the lineside storage containers, the batteries, finessing the train and all those sorts of things. But there was still quite a lot of work to do,” said Green. “Bringing it into Great Western allowed them the time and the space to actually finish the work and get the excellent results they achieved.”
The year-long trial saw the converted Tube trains running a five-mile return trip between West Ealing and Greenford in London’s western suburbs. Usually served by DMUs, the route was an ideal testbed for the converted BEMUs and, more importantly, the fast charging technology that could facilitate battery trains of all stripes across the UK.
“The train is the least interesting bit,” said Green. “The interesting bit is essentially what’s happening lineside, the charging technology, and ultimately what that means in terms of viability of this on parts of the network that are potentially quite remote, where traditionally you would need 132 kV feed somewhere coming into substations to provide 25,000 volt overhead line supply.”
Unlike Alstom’s overhead fast charging system in Drogheda, GWR’s solution uses electrified rails that sit on the track, only becoming live when a train is positioned above. Charging shoes are lowered as a train rides up a short rail approach ramp, making solid contact with the top of the charging rails. Once sensors confirm that everything is correctly in place, the containerised lineside system sends high current to the train batteries.
The trial’s success has delivered proof-of-concept for the charging system and for UK battery trains in general, to some degree. While the trial route was just a five-mile return trip, the resulting GWR whitepaper suggests that battery technology could replace a significant proportion of the operator’s diesel routes. “Although the trial is happening on a Great Western branch line - and we have more branch lines than any other operator in the UK - actually, it starts to look viable for all sorts of applications more widely,” said Green.
It’s still early stages for the UK. Operators will need to work with Network Rail to plan the optimal mix of overhead electrification and fast charging, then tailor procurement accordingly. But from an engineering perspective, the pieces are all in place and battery trains look set to play a big part in the future of Britain’s railways. The relatively distant 2040 date for scrapping diesel means we won’t see entire fleets going electric overnight, but the sector is ready to deliver, according to Simens Mobility’s Clark. “I think if people were ambitious enough to place some orders, then we could probably replace every diesel in pretty short order,” he said.