Chiltern Railways renationalised after 30 years
It joins Great British Railways as part of government plans to save money and improve services.
It is the 10th rail company now under the Great British Railways (GBR), the government's programme of public ownership, which it says will save money and improve services.
Chiltern Railways, which was privatised in 1996, operates services from London Marylebone to Buckinghamshire, Oxfordshire and Warwickshire, as well as destinations in the West Midlands.
The government said the transfer into public ownership would tackle overcrowding by introducing 25 additional daily services from December.
"The most difficult thing on Chiltern is that it's shorter capacity and those extra trains and those extra seats will make a real difference to people travelling on this line up here from Marylebone every day," he said.