IN February this year Britain's Association of Train Operating Companies (Atoc) published its Long-Term Passenger Rolling Stock Strategy for the Rail Industry, which anticipates up to 19,000 new electric vehicles will be required over the next 30 years to accommodate fleet renewal and anticipated growth in passenger numbers.

But even within the next 10 years, Britain faces a significant increase in the requirement for emus. With the electrification of 3000 track-km between 2011 and 2019, NR forecasts that the proportion of electric vehicles in the passenger fleet will rise from 68% today to 80% by the end of the decade.

If the highest level growth scenario outlined by NR becomes a reality, demand for new build vehicles would run at 12 vehicles per week - a challenging prospect for suppliers - and even the more conservative estimates suggest the need for additional electric trains will be significant.

Against this backdrop, and with pressure to optimise the performance and life-cycle of existing assets, leasing companies and train operators are focusing on how they might get more out of the large suburban and regional emu fleets built for British Rail (BR) in the 1980s and 1990s, which remain in widespread use.

For those trains expected to remain in service beyond 2020, leasing companies are investing in modifications to comply with the Persons of Reduced Mobility Technical Specification for Interoperability (PRM TSI) as well as other measures to improve the passenger environment on these trains.

Life extension doesn't end with 21st century interiors, it also encompasses 21st century traction systems.

Earlier this year, leasing company Porterbrook awarded Vossloh Kiepe a £40m contract to supply IGBT three-phase ac traction equipment for the fleet of 91 class 455 emus used by South West Trains on commuter services from London Waterloo.

The 750V dc four-car trains were built in 1982-1985 and refurbished between 2004 and 2008. The fleet is currently undergoing C6 overhauls, which include modifications to comply with the PRM TSI.

Emu-fleetAs the class 455s are considered to have a long service life, Porterbrook and South West Trains (SWT) have agreed to completely replace the dc traction systems in a project which Porterbrook's operations director Mr Alex White says will offer a lower-cost alternative to procuring new trains.

"The business case for re-tractioning is always going to be a more commercially favourable option than buying new trains," White explains. "In this instance the business case is driven by regenerative braking capability and the impact on maintenance intervals and content."

The switch to ac traction equipment and solid state inverters with regenerative braking is expected to generate energy savings of around 20% and will increase the maintenance interval from 16,000km to around 24,000km, freeing up capacity at Wimbledon depot to accommodate expansion of the SWT fleet.

The new traction system will blend the electrical brake on the powered vehicle with the conventional friction braking system across all four cars on the train, thereby extending the life of braking components.

According to Vossloh Kiepe, the unsprung mass of the modified trains will also be reduced, lessening the impact on the track.

The class 455 is one of the most reliable of the ex-BR emu fleets and Porterbrook is eager to maintain, if not exceed, the existing Miles per Technical Incident (MTIN) rate of more than 64,000km, which is defined as an event resulting in a delay of three minutes or more. "We believe we can sustain that level of reliability but there are opportunities for improvement," says White.

Work is due to begin on converting the first train next month at Wimbledon depot, and this unit is due to be completed by the end of May, re-entering passenger service in November. The remainder of the fleet will be converted by February 2016 with the work being carried out at LNWR's Eastleigh facility near Southampton.

Porterbrook says that only two trains will be out of service for conversion at any one time, and installation will take only six days to complete when the programme is in full swing. The conversions will be carried out in parallel with C6 and C4 overhauls to minimise the impact of the programme on SWT operations.

Vossloh Kiepe UK, formerly Transys Projects, is responsible for the design and engineering integration of the new traction system, which will be supplied by its German parent company.

Perhaps inevitably for trains that are around 30 years old, technical variation is one of the challenges engineers will encounter on the project. Each class 455/7 set includes an intermediate trailer from a class 508 emu, a completely different design with a different body profile. White says the refurbishment programme carried out previously has given Porterbrook a head start in this area. "We've done a lot of work on these trains over the years so we have a good idea of what we're looking at," says White. "Our feasibility analysis looked at weight distribution, which will be different on the modified trains, and integration but no problems were found."

Vossloh Kiepe is leading the development of a new safety case for the modernised trains which covers electromagnetic compatibility and the mechanical modifications necessitated by the conversion to ac traction.

Porterbrook is now investigating the possibility of equipping its fleet of 86 class 319 750V dc/25kV ac emus with three-phase traction. These trains are due to be redeployed from the Thameslink route to newly-electrified lines west of London and in northwest England.

Following a feasibility study, Eversholt Rail is also planning to fit Vossloh Kiepe three-phase traction equipment on its class 321 demonstrator set for evaluation.

With an obvious long-term market for these trains, and many franchises up for renewal over the next few years, it looks likely that re-tractioning will be an attractive choice for operators seeking a low-cost option to improve rolling stock performance to meet the demands of a growing railway.