Third rail schemes have high energy losses

Rail line upgrade to be pilot for phasing out 'third rail'

Plans for the UK’s railway network include opening the way for future conversion of ‘third rail’ electrified lines to overhead wires.

The so-called High Level Output Specification (HLOS) for 2014 to 2018 – a formal statement of what the government wants the railway to deliver – includes the creation of an ‘Electric Spine’ through central England, as well as other targeted work, most notably in South Wales, where the previously announced Great Western electrification to Cardiff will now be extended to Swansea and also cover the Welsh Valleys.

The Electric Spine will be a high-capacity passenger and freight corridor electrified at 25kV and linking the core centres of population and economic activity in the East and West Midlands and Yorkshire with the south of England and its ports. This includes electrification of the Midland Main Line from Bedford to Nottingham, Derby and Sheffield, as well as between Oxford and Nuneaton.

Significantly, though, the line between Southampton and Basingstoke, part of the existing third-rail (750V DC) network, will have to be converted to an overhead 25kV AC supply to form the southern end of the Spine. This could be the first step in a wholesale upgrade.

A paragraph in the HLOS document says: “The Secretary of State also wishes the industry to develop a longer-term proposition and business case for the systematic upgrade from DC to AC of the whole Southern network, for consideration for future control periods. As part of this work the industry will wish to treat the conversion work required for Southampton to Basingstoke as a pilot scheme for such a potential modernisation programme, and review plans for renewing or upgrading the DC network to avoid the risk of nugatory expenditure.”

The existing third rail system is characterised by high energy losses, restrictions on train performance and susceptibility to ice and snow.

Conversion to AC would be costly and would have to be carried out in stages, but in fact the industry has been giving it serious consideration recently, not least because much of the DC distribution equipment needs renewing in the next ten years.

In August 2011 RSSB (Rail Safety and Standards Board Ltd) published the results of an investigation into the economics of the 3rd rail DC system compared to other electrification systems, which concluded that switching to 25kV AC “appears to be both feasible and economically desirable,” though the study did not look at the affordability of the change.

About a quarter of DC rolling stock is already dual voltage, and 40 per cent could be converted.

Further information:

DfT: Railways Act 2005 Statement for Control Period 5:

RSSB: Investigating the economics of 3rd rail DC system -

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