Move to modernise four London Underground lines by 2022
A £5.54bn upgrade of four London Underground (LU) lines should increase train frequency and improve efficiency, Transport for London (TfL) has claimed.
District, Circle, Metropolitan and Hammersmith & City lines will receive new tracks, longer platforms, a new signalling system and rebuilt depots.
TfL said passengers would see the benefits on the Circle line by 2021.
But London Assembly member Val Shawcross said the upgrade was now due four years later than first stated.
Once the work is complete, Circle line trains will arrive every four minutes, instead of every 10, and there will also be trains arriving every two minutes on the other three lines, TfL said.
The move will also include 191 new air-conditioned walk-through trains.
Colin Stanbridge, chief executive of London Chamber of Commerce and Industry, welcomed the upgrade.
"This is the sort of targeted investment that is essential for London's transport system to service a rapidly expanding population", he added.
However, Labour's Val Shawcross, the London Assembly transport spokesperson said: "Not only are these upgrades now expected to complete four years later than planned, the new contract comes after the mayor and TfL wasted over £85m of taxpayer money on the disastrous failure of the previous Bombardier deal [to upgrade the signalling on the four LU lines]".
Tfl said the improvements would be delivered within its existing business plan and would provide four pounds worth of economic benefit for every one pound spent.
TfL also estimates capacity on the lines will be increased by 30% and said it was making the changes to cope with the predicted 1.4 million increase to London's population by 2030.
Delays on all lines should be reduced by up to a third by the end of the year, it added.
Tfl said the plans for a new signalling system include the oldest part of the network built, which was built in 1863 and "belong in a museum".
Negotiations are in the final stages with Thales, the French defence firm behind changes to the Jubilee and Northern lines, to make the changes, said TfL.