The Mayor of London joined rush hour passengers on the new London Overground link from Clapham Junction this morning.
It is the first orbital railway to open in the capital in 128 years, with Boris Johnson dubbing it the 'M25 of rails'.
The service launched yesterday and stops at Wandsworth Road, Clapham High Street, Denmark Hill, Peckham Rye and Queens Road Peckham.
There will be four trains an hour from Clapham Junction and Transport for London (TfL) estimate more than 12m people will use the route each year.
Mr Johnson who was joined by Transport Secretary Patrick McLoughlin, said: "People in South London can go to Canada Water and Canary Wharf, where there are employment centres and jobs.
"It is a big step forward for us to integrate rail services in London."
Mr McLoughlin said: "This final connection across South London will be a boom for businesses and commuters alike and I congratulate all those who worked on this successful scheme."
The announcement has coincided with the cutting of the South London Line, removing services from Clapham High St and Wandsworth Road to Victoria.
Eileen Conn, chairman of Southwark Rail Users Group, said: "While we warmly welcome new London Overground service and the benefits it will bring, there can be no escaping the fact that the withdrawal of the South London Line will be a major blow to many, many people."
Mr Johnson said the South London line closure was a decision made by the Department for Transport due to capacity issues at London Bridge.
Claire Shooter, 25, of Tuffnel Park, was travelling on the train.
She said: "My boyfriend lives in Surrey. Before it took me an hour, this will take 40 minutes - it is so much quicker."
Lezleyanne Nabiryo, 22, of Carshalton, was travelling to work and said: "It's better than the underground."
The link has cost £75m, with the Grant Road entrance in Clapham Junction station getting £1.6m upgrade.
A journey from the station to Surrey Quays takes about 24 minutes, while the connection to Canada Water and the Jubilee line takes 28 minutes.