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A previous article in Rail Engineer (issue 157, November 2017) outlined plans for bi-mode fleets on UK railways. This article briefly described Porterbrook Leasing’s Class 319 Flex conversions. A total of 32 units are currently on order, more than half of which will be the UK’s first tri-mode trains (AC, DC and diesel).

Rail Engineer was particularly interested in the engineering and performance challenges of this project and was given the exclusive opportunity to talk with Helen Simpson, engineering innovation and development manager of Porterbrook, and Simon Evans, group innovations director of Wabtec Faiveley UK. They talked about the background and engineering challenges on this ground-breaking project and kindly escorted your writer on a tour of the production facility whilst the first trains were being completed.

MAN D2876.
MAN D2876.

Initial considerations

Class 319 trains were originally provided for the Thameslink route (currently operated by GTR) in the late 1980s/early 1990s. There are 86 units with sub-fleets that have slightly different interior layouts. They are all capable of operation using 25kV AC overhead and 750V DC third rail and can operate at speeds up to 100mph.

The entire class has been through a recent upgrade programme, with the addition of passenger information system displays, as well as controlled emission and universal access toilets. The fleet was withdrawn from GTR as the Class 700 trains were introduced. Some units have been leased to Arriva Rail Northern and West Midlands Trains, operating on 25kV only.

The story starts in September 2014 when Helen Simpson was asked to start looking at how the Class 319 trains might be “re-purposed”. At this time, an ambitious electrification programme was in full swing, but Porterbrook concluded that there would never be a case to electrify all routes and there was likely to be a role for bi-mode suburban/regional trains to allow through journey opportunities, even without continuous electrification.

With the cancellation and delays to schemes since then, the opportunity has expanded.

Of course, there was a lot to consider, evaluate and model before any metal was cut. The key decision was the independent power source. Porterbrook determined that the self-powered performance needed to be at least as good as a Class 150 Sprinter train and that it should have a broadly comparable range.

Alternative power sources such as batteries, flywheels, and supercapacitors were rejected because of their lack of range, and hydrogen fuel cells were rejected because of the novelty risk, lack of hydrogen infrastructure for refuelling, and approvals risks (drafted before the announcement of the Hydroflex, covered in this issue).

Consequently, Porterbrook decided that the only fully developed option was diesel power, delivering the best compromise of range, weight, physical size, power density and total cost.

The engine raft nears completion.

Concept design

After considering a number of supply chain options, Porterbrook formed a partnership with Wabtec Brush to develop the design and convert the units. Brush has expertise in complex engineering design and prior experience of repowering the Class 73 electro diesel locomotives.

The basic concept provides for fitting a diesel alternator power pack to each driving trailer (DT) vehicle, driving the original traction equipment via the existing DC bus line with a return cable added (on electric power the return is via the track). Electronic control units are being provided a) to trick the traction package into thinking it is being powered by a DC conductor rail and b) to control the power output of the diesel engines. Rail Engineer was told that the diesel-electric package is expected to be more efficient than current diesel-hydraulic units during acceleration

The engines are MAN D2876, producing up to 390kW and connected to ABB alternators. The engines are compliant with the EU stage IIIB requirement. They have a selective catalytic reduction (SCR) system to reduce NOx emissions using AdBlue – Helen commented that the engines produce lower emissions than most existing diesel rolling stock and are expected to be quieter. She added that the MAN engine, one of the few that is compact enough to fit under the train, is widely used in rail applications in Europe and is supported in the UK by Wabtec group member LH Plant.


The rated output of two diesel engines at 780kW is the maximum they can produce. By the time alternator efficiency, auxiliary supplies, and traction control efficiency are taken into account, the electrical input to the traction motors will be approximately 550kW, little more than half the maximum rated output of the traction motors.

Simon and Helen are well aware of this deficit. They explained that simple headline figures do not tell the whole story and that the required performance on diesel was likely to be as good as, if not better than the Class 150. Simon described the modelling that had been carried out to gain confidence that the Class 769 trains – the new designation for what had internally been called the Class 319 Flex – will deliver this required performance. He said that Wabtec had constructed its models from scratch and some worst-case routes had been selected to demonstrate the performance.

Modelling is only as good as the quality of the modelling algorithms, data and assumptions and Rail Engineer heard that a degree of conservatism has been built into the modelling, leading to confidence that the Class 769 will out-perform class 15X DMUs on the tough routes modelled.

Compared with a Class 150, the Class 769 has a higher tractive effort on starting, but the tractive effort falls away more steeply. This difference in tractive effort curves makes it difficult simply to predict performance on any particular route, illustrating the importance and value of modelling. Modelling has shown the gradient balancing speed on a flat gradient when powered by the diesel engines to be approximately 87mph and the trains will retain the 100mph capability when powered by electricity. The modelling has also shown that two 1000-litre fuel tanks should be ample for the expected duty.

Whilst maintenance costs will inevitably rise compared with an electric-only Class 319, track access charges should be similar to the donor units. Compared with class 15X DMUs, overall fuel consumption – even on all-diesel routes – and routine maintenance costs will be lower, due to the use of a modern diesel engine requiring less maintenance.

Fitting out the control cubicle.
Fitting out the control cubicle.

Physical modifications

Converting a Class 319 train into a tri-mode Class 769 involves the following on each driving trailer (DT) vehicle, except where stated otherwise:

  • Installation of a new MAN engine and ABB alternator power pack, raft mounted and using similar attachments to those on similar modern DMU rafts such as Turbostars;
  • Installation of the SCR system including an AdBlue tank;
  • Moving the existing heater contactor equipment case to accommodate the new powerpack;
  • Installation of a new exhaust system with a layout that is similar to that on a Class 150, given the similar carbody and bogie configuration;
  • Modification of the power control circuits and traction interface controller for diesel mode (DT, motor second open (MSO) vehicles);
  • Fitting fire barriers, a fire suppression system and a fire alarm system to control the risks of fire on or around the diesel engine equipment;
  • Minor modifications to the cab controls and cab circuit breakers to allow the driver to select diesel mode, AC or DC;
  • Installation of new return cable and jumpers for the 750V DC supply (all vehicles);
  • Fitment of track circuit assistor (TCA), a first on an EMU. It has to be disabled when drawing electricity from the OLE or third rail;
  • For the tri-mode vehicles – fitting shoegear to the trailing bogies on DT vehicles to avoid physical conflict with the TCA and fitting a new power bus to replace the original one which is used by the diesel generator; the additional changeover contactors (diesel to DC) will be fitted to the intermediate trailer car and motor car;
  • For vehicles specified to have air conditioning/air cooling, a static converter will be fitted in place of the motor alternator which has insufficient capacity for the additional load.

This description shows clearly that this is an extensive modification. Simon explained that more than 60 engineers have been involved in the design using more than 45,000 engineering hours to produce over 2,500 drawings, detailing more than 3,500 components for each conversion.

As is often the case with older vehicles produced by what are euphemistically called “coach-built techniques”, the vehicles did not always conform to the drawings and some of the original drawings were missing or incomplete. Access to some of the original designers was a great help, as was the early delivery of a Class 319 to Brush’s Loughborough factory allowing designers to compare drawings with the real thing.

The modifications add approximately 7.5 tonnes to the DT vehicle. In discussion about the impact of this extra mass, Simon revealed that finite element analysis of the bodyshell had been carried out, that the axles are strong enough, and that the suspension and brake cylinder pressures had been adjusted to compensate for the additional mass.

Inevitably, there were design and logistic challenges, such as accommodating an exhaust on a train never intended to have one, even though the design resembles the class 150 exhaust which has similar bogies and carbody. It was also important to make sure the extra mass was evenly spread around the underframe, which in turn led to some space constraints.


Helen talked about the compliance and approval process. The modification is not considered an upgrade or renewal and does not require authorisation under the common safety method for risk evaluation and assessment, although this process has been voluntarily applied as a robust means of managing safety. SNC-Lavalin is providing integrated Notified Body, Designated Body and Assessment Body services.

Full details of this process would justify its own article, and Helen described some of the challenges applying the approval process mandated by the Technical Standards for Interoperability (TSI) regulations on a 30-year-old train. For example, TSI noise requirements do not apply, but pass-by noise will be compared to other DMUs operating the same services on the route; in the case of the Northern trains, this means comparing with the Class 15X units. Porterbrook needs to show it is no worse, but is actually aiming for it to be a demonstrable improvement. This, and other type approval testing, was expected to be carried out at the nearby Great Central Railway.

The completed exhaust stack.
The completed exhaust stack.

The programme

The initial eight Northern units are being completed by Wabtec Brush Traction at Loughborough. During a tour of the factory, a number of vehicles in various states of upgrade were observed, as were the neat modifications to the body to attach the engine/alternator raft. Several engine/alternator rafts and control cubicles were seen under construction (bringing back memories of visits to Brush Traction in the 1980s during the building of equipment cases for the LU 1983 tube stock).

The tour concluded with a visit to the engine/alternator test cells, three of which have been constructed for this project.

The core Flex modifications are not the only works being carried out to these trains. As stated earlier, they have recently had PRM (persons of reduced mobility) works carried out together with the installation of retention tanks for the toilets.

All of the core Flex work will be carried out at Loughborough. For Northern, reliability improvement, re-branding and other modifications are being undertaken by Knorr-Bremse Rail Services (KBRS) under contract to Northern. For the Arriva Trains Wales (ATW) and subsequent trains (such as GWR), all works other than core Flex will be carried out by KBRS. The ATW trains are due for delivery between August and October 2018.

“All works” can include air cooling (which require the static converter), CCTV – both saloon and forward facing, at seat USB and power sockets, Ethernet backbone to support engine control and Wi-Fi, interior and exterior rebranding, and guard’s door control panels.

The first Class 769 unit arrives at Quorn and Woodhouse on the Great Central Railway for dynamic testing in diesel mode.
The first Class 769 unit arrives at Quorn and Woodhouse on the Great
Central Railway for dynamic testing in diesel mode.

Dynamic testing

At the start of the process, Porterbrook resolved to provide trains able to operate on both electrified and non-electrified networks with a simple switchover, potentially on the move. Performance in diesel mode was to be similar to Class 150, uphill with several stops, and a refuelling range of at least one day (500 miles). This was to be accompanied by diesel engine fuelling and maintenance similar to Class 150 and a driving style as close to Class 319 as possible in order to deliver a bi-mode train that was attractive to operators.

Helen said that, subject to test, all this has been achieved and she is looking forward to seeing the results of the dynamic testing.

On 14 September, the first Class 769 unit arrived at Quorn and Woodhouse on the Great Central Railway to commence that dynamic testing. The diesel-powered aspects of the upgraded train will be put through their paces over the coming weeks.

The Future

Helen and Simon indicated that other orders might be forthcoming but would not be drawn on what they might be. They also indicated the possibility of other Flex products, possibly a true hybrid design based on the Class 455 750V DC units. These have been extensively refurbished and have been retrofitted with three-phase AC traction systems incorporating regenerative braking. There would be space on the intermediate trailer coach for batteries that could be charged by the regenerated energy and by the diesel engines. Such a feature could have several benefits such as being able to stop the diesel engines in terminal stations and to supplement diesel engine power when accelerating.

Thanks to Helen Simpson, Porterbrook and, Simon Evans, Wabtec for their tour of the production facility and for patiently answering questions, and to Rupert Brennan-Brown and Karen Jackson of Porterbrook for facilitating the article.

Photo: Jamie Steel.
Photo: Jamie Steel.

Class 769 orders (as of May 2018):

On 22 December 2016, Northern (Arriva Trains Northern) and Porterbrook Leasing announced that they were cooperating to extend the reach of the Class 319 trains that have been leased for the North West Electrification programme. Eight of their four-car units were to be converted into bi-mode trains, described as Class 319 Flex.

Abellio Wales & Borders and the Welsh Assembly Government announced in July 2017 that they would take five Class 319 Flex trains, operated in diesel mode, to cover for Sprinter DMUs that needed to have modifications carried out to conform to the Technical Specification for Interoperability – Requirements for People of Reduced Mobility.

Next, on 23 April 2018, Great Western Railway announced that it had ordered 19 of the trains, now known as Class 769 Flex; these will be tri-mode units (diesel and AC/DC) fitted with air cooling, at-seat power sockets and Wi-Fi.

Read more: Velaro Novo: Siemens’ new approach to high-speed travel


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. The Class 769 on Great Western Railway will replace the Class 165 & Class 166 on the Reading-Gatwick Airport service with some transferred to operate on the Thames Valley local lines and the rest to operate elsewhere on GWR. Plus some might even start from Oxford and Swindon to Gatwick Airport.

  2. Horrible trains but it’s a smart concept. They would be much improved with a better interior layout with more seating bays aligned to the windows.


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