Passengers numbers have grown by more than 160% since Transport for London took over the under-performing Silverlink service in 2007.
The capital’s growing population and the recent opening of an extension at Clapham Junction are expected to see number rise even further.
Mayor Boris Johnson and TfL have now announced plans to boost the number of carriages on trains from four to five, adding capacity for a further 150 passengers per journey, and to extend platform lengths.
A total of 57 extra carriages are expected to be in service by the end of 2015.
The Overground is consistently one of the UK’s best performing rail services and scores highly on passenger satisfaction league tables thanks to its modern trains and visible staffing.
Mayor Johnson, TfL, London Assembly Members and passenger watchdog London Travelwatch have all called for devolution of further routes to allow even more passengers to enjoy the high-quality, metro-style service.
Announcing the plans to boost capacity, Mr Johnson said: “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.”
Travelwatch chair Stephen Locke added: “We welcome this latest investment to provide longer trains. London TravelWatch strongly supports London’s Mayor having a greater say in running the capital’s national rail services and these recent improvements are just one example of how passengers can benefit when this happens.”