Atomic has been awarded the creative account for East Midlands Railway, the UK rail franchise run by Dutch business Abellio Group.
The agency, which already works on Abellio-owned rail franchise Greater Anglia, won the business after pitching against a number of agencies in a process managed by The How to Buy a Gorilla Company.
Abellio took over East Midlands Railway last summer, when it promised £600m of investment during the eight-year contract. The company won the contract after Stagecoach, which had managed the franchise since 2007, was disqualified from bidding after refusing to take on pension liabilities.
Atomic will work on a creative brand platform for the brand, aimed at boosting awareness and creating a "strong emotional connection" with its target market.
Abellio has said it will provide the franchise with new trains, 165 new carriages and better infrastructure. East Midlands Railway was known as East Midlands Trains until it was taken over by Abellio, which itself is owned by Dutch state-owned company Nederlandse Spoorwegen and also operates ScotRail.
The One Off handled the ad account when it was East Midlands Trains.
East Midlands Railway runs services between London St Pancras and Sheffield, Nottingham, Derby and Leicester.