In 2017 the Secretary of State for Transport, Chris Grayling, cancelled the Midland Main Line electrification and curtailed the Great Western electrification because costs had escalated to an unacceptable level, driven by arbitrary dates set by the DfT and a lack of electrification design experience, which had resulted in significant design, specification and implementation mistakes and a failure to use risk mitigation processes to achieve sensible low cost solutions.

Network Rail is completing the other, smaller electrification schemes which were in progress but the only new scheme being planned is the TransPennine route between Stalybridge and Leeds. Despite the lack of commitment to electrification, in 2018 the then Rail Minister Jo Johnson set the target of phasing out diesel-only trains by 2040 to decarbonise the railway.

Graphic from RSSB Options for Traction Energy Decarbonisation report This graph, taken from the RSSB Options for Traction Energy Decarbonisation report, shows the amount of CO2 produced per unit of energy for a range of possible fuels. The two figures for overhead electrification and hydrogen by electrolysis are for the current (2018) and predicted (2040) energy source mix for electricity generation. Green hydrogen is produced by electrolysis using renewably-generated electricity (eg wind power) at source.

Surplus electric trains

The objectives of increased quality and capacity set by the government for new franchises have resulted in the winning bidders ordering new trains, so rendering many existing electric trains surplus. With no further electrification schemes in prospect, the rolling stock companies are proposing conversions of these EMUs to battery or hydrogen power – although new build units could be lighter and therefore use less energy and be less polluting. Vivarail is also building diesel, diesel/battery hybrid, battery/electric and hydrogen versions of its D-Train, based on redundant London Underground District Line stock. The DfT’s Rolling Stock Perspective suggests that these trains could strengthen the case for reopening disused lines.

Discontinuous electrification

The government has instructed Network Rail to consider discontinuous electrification to reduce the cost of electrifying the TransPennine route. This technique was used for the electrification to Paisley Canal, where by adopting a lower than normal wire height and inserting a neutral section under one of the bridges, the cost lowering track or raising bridges was much reduced. Electric trains coast through the neutral section, which is not near a station. However, due to the low wire height only multiple units can run whilst the overhead is energised; it must be de-energised for freight trains to operate.

The TransPennine route carries a lot of freight and has several long tunnels, so the option of reducing the wire height is not possible and discontinuous electrification would require long neutral sections. Therefore electric trains would require an alternative source of power such as batteries for the neutral sections. This would add to the weight of the trains and reduce the power available, severely compromising the key reason for electrification, namely quicker journeys over the Pennine gradients needed for the economic prosperity of the Northern cities, which only full electrification can deliver.

Funding for alternative technologies

The Innovate UK funding competition winners were announced in January 2019. The government will provide initial funding for five schemes to reduce the carbon footprint of the railway. One is for Riding Sunbeams, a scheme to use solar power for rail traction. A second scheme is for a different way to use hydrogen to power trains, by combining it on-train with oxygen to produce high pressure steam which will drive a turbine to generate electricity to power the train. The other three schemes are for different ways of making diesel engines more efficient and therefore reduce carbon emissions.

Diesel-electric bi-mode

Roger Ford notoriously described bi-modes as over-weight electric units or under-powered diesel units. They are best used to enable through services when part of the route is already electrified, whilst the remainder is being electrified. However, the DfT seized on them as a way out of the escalating cost of GW and MML electrification, claiming that the same time savings could be achieved with bi-modes on MML without the capital cost. The time saving will actually be achieved by removing intermediate stops, so the service for passengers will be worse, whilst the operating and maintenance costs (for both track and train) will be significantly higher. The increased weight results in higher energy consumption, and therefore emissions, than either electric or diesel trains.

The engines on the bi-mode class 802 add 8 tons each, a weight penalty of 11% compared to the Class 801 EMU. The power available from diesel is also less than when running on electricity from the overhead lines. This reduces acceleration on diesel by one third compared to electric power, hence the need to remove intermediate stops to achieve the promised schedule on MML.

Porterbrook are converting class 319s to class 769 diesel-electric bi-mode. Testing has taken a lot longer than expected, so the dates quoted for battery and hydrogen conversions may be similarly optimistic. The power/weight ratio of a class 769 on diesel power will be less than the class 150 or class 166 that they will replace. In addition Northern drivers have found that the class 319s struggle during leaf-fall due to poor adhesion (the powered axles are all on one coach) so it is likely that this will also occur on the class 769 conversions. These issues could have implications for timekeeping.

Stadler have a different design concept for their diesel-electric bi-modes which will be introduced by Greater Anglia; instead of underfloor engines, which limits the power available, the engines will be in a separate power car which could be removed from the unit to convert it to a pure EMU. However, the use of articulated bogies increases load on the track which means that speed will be limited on many lines, again affecting journey times.

Diesel-battery hybrid

This is the rail equivalent of a Toyota Prius. Rolls Royce are offering hybrid powerpacks which would replace the existing diesel engines. This will be tested on one of Porterbrook’s class 170 diesel multiple units. The powerpack contains a diesel engine which would run at its most economical speed, driving an electrical machine which acts as a generator to charge a battery pack when the train is at constant speed, braking or stationary, and which acts as a motor using energy from the battery pack to provide the extra power required for acceleration. The engine would not be subjected to the on-off duty cycle of a normal diesel, so wear would be reduced and there would be a 25% fuel saving. Therefore operating costs and emissions would both be lower than a conventional diesel.

Vivarail are building diesel-battery hybrid trains for Transport for Wales, using the same components as their diesel and battery prototypes.


Battery powered trains are nothing new; London Underground has been using battery locomotives to haul engineering trains for many years, and in the 1950s British Rail experimented with a battery multiple unit. However, it is only with recent advances in battery and battery charging technologies that battery power can offer the range, performance and battery life necessary for passenger services.

Class 379 battery pilot.  Photo by Network Rail Class 379 pilot. Photo by Network Rail.

In 2015 Network Rail trialled a class 379, converted by Bombardier to run on battery power or overhead, on the Harwich branch. The duty cycle for the trial was 50km OLE and 30km on battery. The battery pack weighed 8 tons, a 5% weight penalty for the unit. The maximum range achieved on battery was 48 miles and the acceleration 0.5m/s/s, equivalent to a DMU. The battery was overstretched for this application, getting too hot, which might have prevented it achieving its target life of 5 years.

The same battery has been used by Vivarail in its prototype battery unit, which is much lighter than the class 379 so here the battery is not overstretched. Vivarail claim a maximum range of 80 miles and a realistic range in service of 40 miles. A 10 minute charge at each end of a 30 mile journey is sustainable, and a battery life of 7 years is warranted by the manufacturer provided the battery is operated within specification, eg for temperature, charging and discharge rates and minimum remaining charge.

The efficiency of energy storage in batteries is between 80-90%. With the weight penalty and the cost of battery replacement, battery trains are more expensive to operate than overhead electric trains (but do not require capital investment in overhead wiring) but cheaper than diesel (and with lower emissions). They are therefore the most economical and environmentally friendly option for services on short light-used lines – it is not even necessary for any part for the service to be on electrified track if charging stations are installed at each end of the journey.

Porterbrook proposes to convert redundant Class 350/2s into battery-electric units when they are sent off-lease from West Midlands Trains.


Hydrogen is being proposed by the government as an alternative to diesel because it is non-polluting at the point of use – the only exhaust product is water. A fuel cell on the train generates electricity directly from hydrogen. Because the power output of the fuel cell cannot be varied quickly to match demand, batteries are used to store the electricity, smoothing the demand. The Government is also funding development of a proposal to generate steam from stored hydrogen and oxygen, and then generate electricity with a steam-powered turbine.

Hydrogen is produced as a waste by-product from some industrial processes in both north-east and north-west England. Hydrogen is difficult to store and transport, so it is proposed that the prototype hydrogen-powered trains will be trialled on lines in those areas. Tees Valley Combined Authority have awarded seed funding for a bid to use waste hydrogen from the chemical industry to power trains and road vehicles in the area.

However, the amount of hydrogen available as a by-product would not be sufficient for widespread use nationally to power rail and road transport, so a production capability would be needed, either by a chemical process from a feedstock such as natural gas or by electrolysis of water. Therefore hydrogen can considered either as a fuel, or as a means of storing electricity – but whereas battery storage is 80-90% efficient, hydrogen storage (production of hydrogen by electrolysis then generation of electricity by a fuel cell) is only 40% efficient, so the energy costs and emissions of hydrogen-powered trains will be double those of battery trains.

The environmental credentials of hydrogen depend on the feedstock used or the source of the electricity for electrolysis – overall CO2 emissions for hydrogen derived from natural gas are slightly lower than for diesel power, but no better than a diesel hybrid, whilst electrolysis using the current mix of UK-generated electricity is as bad as pure diesel, although by 2040 that should have improved to be better than a diesel hybrid – but not as good as overhead electrification. Unless a cheap source of electricity is available, the cost of hydrogen power is likely to be higher than diesel.

Alstom and Eversholt are working to develop a hydrogen-powered 3-car version of the Class 321 Renatus, named the Breeze, for introduction to service by 2022. Whilst the Coradia iLint train developed by Alstom for use in Europe will have roof-mounted hydrogen tanks, the British loading gauge prevents this approach so the tanks will be mounted in a section inside each of the Class 321 driving coaches.

Porterbrook are working with Birmingham University to develop a prototype hydrogen-powered class 769 for demonstration runs in 2019. Like the bi-mode version, the reduced power available away from the wires will limit this to a maximum speed of 87mph.

Vivarail is also developing a hydrogen-powered version of its D-Train, with the hydrogen tanks and fuel cell under the centre car and batteries under the two driving cars.

Overhead electrification

The government cancelled electrification of the Midland Main Line because costs had risen to unacceptable levels, driven by arbitrary dates set by the DfT and a lack of electrification design experience, which had resulted in significant design, specification and implementation mistakes. At the time, Railfuture argued that the costs seen on GWML need not be repeated, that Network Rail should use the breathing space which bi-modes offer to identify and address the reasons for these cost increases, and that a rolling programme of incremental electrification is essential for NR to act as an informed, intelligent client, learning from each project and applying those lessons to the next.

Photo by Network Rail Electrification in progress at Bristol Parkway.

Electric trains are cheaper to operate than diesel - the purchase costs are lower, they are more energy-efficient, and they cost less to maintain. They are more energy-efficient than the alternatives above, and offer the possibility of zero emissions if electricity is generated sustainably. However, overhead electrification is capital intensive, so there is a trade-off between the capital costs and the operational cost savings which depends on the intensity of traffic on the route and the proportion of each journey that diesel trains travel on electrified track. Railfuture’s analysis shows that to achieve a 2:1 benefit/cost ratio with an efficient capital cost of electrification versus the opex costs of diesel operation, there need to be 24 vehicles per hour each way on double track if none of the route is electrified, but only 12 vehicles if the route is already 50% electrified, such as on the Midland Main Line – and this is before taking into account other benefits such as journey time improvements, the additional ridership that this will generate, and reduced emissions.

On 14 March 2019 the Rail Industry Association published its Electrification Cost Challenge report which shows that electrification projects can be delivered for between £750k and £1m per single track kilometre, ie between 33% and 50% of the £2.5m per single track kilometre cost of GW electrification. The report confirms our view that a rolling programme of electrification with achievable dates, at a level which can be resourced and managed effectively, would enable the rail industry to build the necessary expertise and progressively lower the long-term operating costs of the railway towards European norms.


A rolling programme of electrification is essential to achieve journey time savings, lower operating costs and reduced emissions on intensively used or high speed routes. However, the alternatives to full electrification have a place on lightly used routes where electrification is not financially justified.

Bi-modes, diesel hybrids, and improved diesel efficiency are ways of buying time, not a permanent solution. Bi-modes should be used to run services which extend off the electrified network whilst waiting for the electrification to be extended. In the short term advanced diesel hybrids can be brought into service sooner and offer lower operating costs and lower emissions than hydrogen power, which requires development, both of the technology and of production and distribution.

Battery-electric trains offer operating cost savings (and lower emissions) against diesel power without a high capital cost, can be brought into service relatively soon, and are effective for low-intensity services of up to 30 miles.

The viability of hydrogen power has yet to be proved. It may have potential for low intensity services over longer distances, but may have higher operating cost than diesel, whether emissions are any lower depends on how the hydrogen is generated, and will require significant investment in production and distribution to become a major energy source for rail – which depends on take-up for road transport.

RIA Electrification Cost Challenge report

Department for Transport Rolling Stock Perspective

RSSB Options for Traction Energy Decarbonisation report

Innovate UK’s competition to integrate proven technologies into the UK rail industry.

Institute of Mechanical Engineers report The future for hydrogen trains in the UK

Why electrify? article