Work is set to begin on stations to accommodate longer London overground trains.
The longer platforms are to make room for longer overground trains, which are being increased from four to five carriages.
Longer trains will first run on the Highbury & Islington to Clapham Junction route from the end of 2014, while they will run on the Clapham Junction to Stratford route by the end of 2015.
The work forms part of an upgrade programme to increase capacity on the trains by 25 per cent to meet increasing demand.
TfL is investing £320m on the improvement programme, with it being a year since the network went live in Clapham Junction.
Mike Stubbs, director of London Overground, said: "Although still in its infancy, London Overground services have proved to be hugely popular with passengers, with numbers quadrupling during the first six years of service.
"As London’s population continues to grow, so too will the demand on London Overground, which is why we’re carrying out vital work such as this to help provide the extra capacity that will be needed."