HomeRail ProjectsClass 769 Flex in Action
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On a sunny afternoon in early November, Rail Engineer travelled to Rothley station on the Great Central Railway, at the invitation of Porterbrook Leasing, to see and hear the Class 769 Flex.

The article in issue 168 (October 2018) celebrated the work that Porterbrook and Wabtec Brush have put into this project, but this visit was arranged to experience the unit in action. How would the engines perform? How much noise and vibration would there be?

There was no need to worry; just walking through the car park with the train alongside was a revelation. The two idling MAN diesel engines were almost purring; none of the ‘rattling’ that one is used to from older diesels and no visible exhaust either. A conversation at normal volume was easily possible, sitting on the benches outside the café just four metres away from the train.

Inside the driving cars, fitted with the diesel alternators, it was a similar story. One is aware of an engine running but it could not be described as noisy. During acceleration, the sound is purposeful, but there was no significant vibration transmitted through the floor and the main noise source was airborne, coming through a hopper window left open for an instrumentation cable. Normal volume conversation was easily possible.

Next steps

The demonstration was almost an anti-climax – testament to the quality of the design and development process. Of course, there have been some minor issues to resolve, but Helen Simpson, Porterbrook’s innovation and development manager, said that they were trivial and vindicated the hard work carried out in Brush’s test cell. Helen was also full of praise for the many UK engineering companies that have contributed the 6,500 items required for each unit.

In discussion, Jonathan Wragg, Porterbrook’s Flex programme director, outlined the production programme. The first unit for Arriva Trains North is due for delivery in January 2019, for Transport for Wales in spring 2019 and deliveries to GWR should start in summer 2019 and be completed in early 2020.

Discussion inevitably turned to how the Flex concept might be extended. Rupert Brennan-Brown, Porterbrook’s head of communications and engagement who is always ready with a new acronym, described Porterbrook as a rolling stock asset management company that is adapting trains to accommodate the inadequacies of the infrastructure.

Summarising previous announcements, developments include:

HydroFlex: A hydrogen-powered train being developed in partnership with Birmingham University, using some of the control technology developed for the diesel electric Flex;

HybridFlex: Working with Rolls Royce (MTU) to provide diesel/battery electric drive for class 168/Turbostar including energy harvesting in braking;

BatteryFlex: providing ‘last mile’ battery power on Class 350/2 Desiro units.

There are 86 Class 319 four-car units, all of which were made redundant from the Thameslink route.  Porterbrook has been successful in placing approximately 45 units for further use – 32 for Northern (eight of which will be converted to Flex specification) and 13 units with West Midlands Trains. In addition, there are Flex orders for five units for Wales, 19 units for Great Western Railway and one for the University of Birmingham (the HydroFlex). This makes a grand total of 71 of the 86 units, leaving 15 still to find new homes.

Wider problem – and opportunity

This is certainly a success story but is the tip of a large iceberg. With the boom in new trains coming into service, Porterbrook alone will see over 200 electric multiple units (around 900 vehicles) of Classes 323, 350/2, 455, 456 and 458 coming off lease over the next two or three years with no immediate home. The other large rolling stock leasing companies (ROSCOs) will be in the same, or similar, situations.

All of these units could be converted to a Flex format. For example, the re-tractioned South Western Railway Class 455 units could take advantage of their regenerative braking capability to have a diesel/battery Flex arrangement. Equally, it would be possible to provide Battery Flex capability on the Class 323.

Many operators on the non-electrified railway have struggled to expand capacity because of the shortage of self-powered trains. Innovations such as Flex are now providing the opportunity. There will be a large number of redundant electric vehicles with useful life left in them, but placing this large group of vehicles into the relatively smaller pool of self-powered trains will be a big ask. That said, based on the Flex experience, this writer would rather travel on a Class 769 than on a Class 150.

Malcolm Dobell BTech CEng FIMechE
Malcolm Dobell BTech CEng FIMechEhttp://therailengineer.com
SPECIALIST AREAS Rolling stock, depots, systems integration, fleet operations. Malcolm Dobell worked for the whole of his 45-year career with London Underground. He entered the Apprentice Training Centre in Acton Works in 1969 as an engineering trainee, taking a thin sandwich course at Brunel University, graduating with an honours degree in 1973. He then worked as part of the team supervising the designs of all the various items of auxiliary equipment for new trains, which gave him experience in a broad range of disciplines. Later, he became project manager for the Jubilee Line’s first fleet of new trains (displaced when the extension came along), and then helped set up the train refurbishment programme of the 90s, before being appointed Professional Head of Rolling stock in 1997. Malcolm retired as Head of Train Systems Engineering in 2014 following a career during which he had a role in the design of all the passenger trains currently in service - even the oldest - and, particularly, bringing the upgraded Victoria line (rolling stock and signalling) into service. He is a non-executive director of CPC Systems, a systems engineering company that helps train operators improve their performance. A former IMechE Railway Division Chairman and a current board member, he also helps to organise and judge the annual Railway Challenge and is the chair of trustees for a multi academy trust in Milton Keynes.


  1. Eversholt Rail are to convert the 40 year old Class 321 EMU units into Hydrogen and possibly Bi-Mode units. When the Class 321 are replaced by the Bombardier Class 720 Aventra as Greater Anglia have ordered which will take over the Class 321 duties on the Great Eastern routes and Class 317 & Class 379 on West Anglia and Stansted Express routes. With the possibility of converting the Class 317 into either Bi-Mode or BEMU units.

    The Class 379 Electrostar built by Bombardier could have batteries fitted as what “379013” was being used known as a BEMU test train and was trialed and tested on the Manningtree-Harwich Town route (Mayflower Line) in Essex some years ago.

    The same way what Porterbrook are to install battery components onto the Siemens Class 350/2 Desiro EMUs which West Midlands Trains are replacing the Class 350/2 with the Class 730 Aventra also built by Bombardier which will come into service on London Northwestern Railway services from 2020/2021.

  2. When the Class 769 Bi-Mode & Tri-Mode units are in service on Northern, Great Western Railway and KeliosAmey Wales (TfW Rail). These will be ideal to operate on busier routes as well on rural lines. Which is why they are so reliable they don’t deserve to be sent for scrap. And to last for at least 25-30 years.


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