Winning manufacturer will be selected at end of month, while construction bidders prepare to submit offers in March.

The race for the £100m bid to supply trains for the £900m East London Line tube extension has hotted up with the shortlisting of four bidders.

Manufacturers Siemens, Bombardier and Hitachi are offering to build new trains, which would be a cross between those used on the Underground and those operating on the rail network. The fourth bidder, the train-leasing company Portabrook, would supply pre-owned trains.

Transport for London will select the winning bidder at the end of next month.

The East London Line is being extended in the north to Dalston and in the south to West Croydon and Crystal Palace in the first phase of expansion, scheduled to be completed in 2010. It is the largest transport infrastructure project in the London Mayor's five-year plan.

Four consortia are already bidding for the main contract, worth around £500m, to build new bridges and stations and lay new track. Those bids are being led by Laing O'Rourke, Mowlem, Balfour Beatty and Skanska.

The construction consortia, which have already been through a pre-qualification process, will submit their offers at the end of March.

The second phase of the expansion will see the East London Line extend across south London to Peckham Rye and terminate at Clapham Junction, mostly using existing track.

This had been slated to be completed by 2012. But the timetable is likely to slip as other infrastructure projects, which will more directly benefit the Olympic Games, take priority to be completed in time for 2012.

Transport for London will receive a £400m low-interest loan from the European Investment Bank because the work will benefit deprived areas of inner London. Funding has not yet been secured for the second phase of the expansion.