Network Rail, the not for profit company which runs the UK's rail infrastructure, has so far used a roster of agencies on its account but is now looking to consolidate the business.
The chosen agency will be working on offline and online activity including direct marketing, loyalty programmes and sponsorship.
The tender is for a three year contract with a focus on London St Pancras and will be overseen by head of marketing services Mark Shaoul.
Network Rail will be looking for agencies to offer in-house media planning and buying or ones that have a media agency partner.
At the end of last year Network Rail called a pitch for design agencies to work on a rebranding project.
The branding project is not expected to change Network Rail's name or logo, but it will involve positioning, values, guidelines and communications platforms.
Network Rail is currently running an above-the-line campaign promoting safety at level crossings, created by Iris London.