Meet the FLEX Family

Innovation rolls out to meet the future

In-line with its commitment to use innovation to tackle railway infrastructure challenges and improve the industry’s environmental credentials, Porterbrook is leading the development of a first ever UK retrofit project to convert electric multiple unit Class 319 units into electric-diesel bi-mode units. As the original donor vehicles are fitted with both 3rd rail and 25kV equipment, the FLEX units can become tri-mode trains and even incorporate hydrogen fuel cells.

Introducing the FLEX concept

The FLEX concept involves converting existing Class 319 units to Class 769 FLEX units, delivering lower operational costs on both electrified and non-electrified routes, by fitting two new powerpacks – one under each driving vehicle. These low emission (Stage IIIB compliant) MAN engines are coupled to ABB alternators which in turn provide power to the existing DC bus, and the traction and auxiliary equipment.

The modular design of the powerpack rafts means the vehicle can be adapted for future energy modes. Diesel engines can be replaced by battery packs and hydrogen fuel cells to create a zero emission self-powered unit.

This exciting concept will result in passenger trains offering performance that can match or exceed the diesel fleets they replace, significantly raise air-quality, and offer a cost effective alternative to commissioning brand new trains.


In September 2018, Porterbrook paired up with the University of Birmingham’s Centre for Railway Research and Education (‘BCRRE’) to create HydroFlex - the UK’s first hydrogen powered train. It is Porterbrook’s and BCRRE’s innovative response to the governments challenge to remove diesel only trains from the network by 2040.

Porterbrook has provided a ‘Class 319’ electric unit to BCRRE for conversion into a hydrogen powered train.  This will allow both organisations to demonstrate how this fuel-of-the-future might be deployed across the UK’s rail network. Testing and demonstration runs are scheduled for summer 2019.

While the HydroFlex retains the ability to operate across existing electric routes (on either 3rd rail or 25kV overhead power), the addition of a hydrogen fuel cell will make it capable of operating in self-powered mode, without the need for diesel engines.

"I am delighted that BCRRE is working with Porterbrook on this revolutionary fuel-cell project. The HydroFlex demonstrates how the private sector railway and educational bodies can jointly deliver real benefits to passengers, train operators and communities."


In October 2018, we took the decision to convert of one of Electrical Multiple Units (EMU) into a battery/electric bi-mode. The class 350 EMU is one of Britain’s most reliable trains, with Porterbrook’s 350/2 version recording 100,420 Miles per Technical Incident.

With the addition of the latest battery technology, Porterbrook is confident that the 350/2 BatteryFLEX could match, or outperform, diesel trains on existing non-electrified routes. The introduction of BatteryFLEX will further reduce carbon footprint and emissions, whilst maintaining performance and delivery.

Porterbrook has modelled energy and power demands for routes such as the Windermere branch, where a BatteryFLEX version of the Class 350/2 fleet would eliminate diesel use on non-electrified lines, and maintain the ‘beneath the wires’ capability of these highly reliable trains.

Porterbrook is currently working with specialists in battery chemistry to create a demonstrator unit for interested train operators, their passengers and the communities they serve.

Porterbrook's FLEX customers

Supply chain partners