Network Rail is moving its mobile connectivity contract from Vodafone to O2 in a move it claims will improve its mobile working productivity and cut costs.
The decision will see 23,000 mobile connections move over to O2, covering voice call accounts, Blackberry e-mail connections and other mobile data services.
Network Rail, which owns and operates Britain's rail infrastructure, is O2's largest corporate customer win.
O2 says it will help Network Rail identify areas of the business that will benefit from mobile technology to enhance efficiency and flexibility for the workforce, and put in place systems to achieve both savings and customer benefits.
Network Rail's director of information management, Catherine Doran, said, "We are keen to embrace new ways of communicating with our people and suppliers and changing the way we work by using new technology.
"O2 has demonstrated the same passion and drive to help us achieve these bold targets, by showing great innovation, good customer service and commercial capability."
The value of the Network Rail contract has not been revealed. Other major O2 corporate clients include Merrill Lynch, the AA and the Royal Bank of Scotland.