Transport for London has confirmed plans to lengthen all trains on the London Overground from four carriages to five.

The £320m programme includes the construction of longer platforms at some stations and the delivery of an extra 57 carriages by the end of 2015.

The heavily constricted Rotherhithe and Canada Water stations are unlikely to be expanded so trains will used selective door opening (SDO) at these stations.

This means passengers in the rear coach of a five-car train are likely to have to walk through to the next coach in order to alight.

At Surrey Quays there is more space for potential platform extensions.

“Passenger numbers on our London Overground services have been going through the roof, but this important programme of investment will enable us to provide the extra carriages required for Londoners to take advantage of what has become the most popular suburban railway in the country,” said Boris Johnson, Mayor of London.

Transport for London’s Howard Smith said: “Demand for London Overground – the UK’s most punctual rail service – is growing at an incredible pace.  By the end of 2012, London Overground carried 120 million passengers – nearly four times the number carried when we launched in 2007.

“To enable this positive trend to continue and maintain our high levels for safe, reliable and frequent travel, we need to deliver more capacity by adding more carriages”.