Battery powered trains

Battery powered trains could allow electrification on some parts of the network where overhead 25kV power is hard to achieve. GWR and Network Rail are testing this technology on a branch line near London. They invited us to see the results.

The problem

Much of GWR’s network is served by old diesel trains which will reach end-of-life by 2035. In order to meet emissions requirements, the new trains that replace them can’t be diesel powered.

For the core network, 25kV overhead electrification (overhead line equipment, or OHLE) still makes economic sense. Externally-powered electric trains are lighter than diesel trains, so they accelerate quickly and place less wear and tear on the track. They are also cheaper to maintain. The upfront cost of OHLE is high, but over time it pays for itself.

Away from large centres of population however it gets increasingly expensive to lay on the kind of power supply required for overhead electrification. When this is added to the cost of making necessary changes to bridges, tunnels and lineside structures, it becomes hard to make a business case for OHLE. A compromise is needed.

Battery-Electric Multiple Units

GWR is currently focusing on battery-powered trains (Battery-Electric Multiple Units, or BEMUs) which can be recharged rapidly at stations. The 4km branch line from West Ealing to Greenford was identified as a suitable place to try out this technology. A test train has been running there since March 2024

GWR recently invited Bristol Rail Campaign to West Ealing to experience this train in action. During our ride from West Ealing to Greenford and back their team explained the system.

How it works

At the heart of this technology is DC fast-charging. This uses a large trackside battery pack which is trickle-charged from a 14kW supply. In practical terms this means that trains can be charged anywhere there is space for the battery pack together with grid capacity for the equivalent of a single dwelling house.

The charging rails are installed between the running rails. They only become live when the train’s charging shoe is connected and the train has completely covered them. The charging shoe is designed to clean the rails, ensuring a good connection.

The test train has just over 10 tonnes of batteries on board, with a capacity of 504 kWh. The train has been tested on the main line, where it ran for over 110km using only 45% of its battery capacity. The charging setup on the Greenford Branch operates at 700 kW, and can top up the batteries in the time it takes the driver to change cabs at the end of a run. The charge rate and battery capacity can be adjusted dependent on the specific application.

GWR claim that the running costs for a two-car BEMU are £2.20 per mile. This compares with £4.15 per mile for hydrogen fuel cell power, and £4.01 per mile for diesel power. According to the team, hydrogen has no place in transportation!

Project Churchward and Discontinuous Electrification

Project Churchward is a joint initiative between GWR and Network Rail to develop a ‘track and train’ solution to replace the ageing DMU fleet. The electrification strategy is central to this project.

BEMUs are likely to be part of this strategy. But in many places hybrid trains using batteries and OHLE may be more practical. This approach, known as Discontinuous Electrification (DE), is seen as a way to avoid some of the costly aspects of overhead electrification. It does this by leaving gaps in the wires where it is difficult to install OHLE. Trains charge their batteries while ‘under the wires’, and then use this battery power to get the to the next section of OHLE.

The current fleet of Class 80x Hitachi trains give a flavour of how this could work, switching to diesel where there is no OHLE. Hitachi are currently developing a version of the Class 80x which swaps the diesel engines for batteries. They claim this will have a battery range of 100km.

DE is currently being considered for East-West Rail, the project to re-connect Oxford to Cambridge by train.

Compromise

Battery-Electric Multiple Units and Discontinuous Electrification are both compromises. In a perfect world all lines would have 25kV overhead electrification, with its many advantages. Batteries increase the weight of trains, making them slower to accelerate and increasing wear on track and train.

We believe that there is still a case for installing OHLE into Bristol Temple Meads, Filton Bank and other main lines in the region. But given the size of the task of decarbonising the railway, and the timescale within which an answer needs to be found, DE and BEMU technology will almost certainly be part of the bigger solution.