Contract will be put out to tender in September but sources have revealed the firms that have passed pre-qualification bids

A shortlist of four bidders are in the frame for a 拢60m contract to renew power supply systems on the London underground as part of an upgrade to the District line.

Transport for London (TfL) is due officially to put the contract out to tender in September. However, industry sources have revealed the firms that have passed pre-qualification bids.

The shortlist of bidders is Balfour Beatty; a joint venture between ABB and Bam Nuttall; UK Power Networks; and Murphy Construction in partnership with Siemens.

The work is seen as an essential step in the process of increasing capacity on the network and is scheduled to begin later this year, with completion required before the arrival of 80 new trains on the line in 2013.

The upgrade of the District line involves increasing capacity by 24% to enable the carriage of 10,000 more passengers per hour. There will also be a new signalling system installed to ensure trains run faster and with fewer failures.

TfL鈥檚 underground upgrade programme will boost capacity by 30% across the network.

Future plans include station rebuilds at Victoria, Bond Street and Tottenham Court Road.

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