It is the first time Network Rail has allowed advertising in its washrooms.
The A3 panels will allow brands to advertise on sites such as the backs of toilet doors, the back wall of some cubicles and above hand dryers and urinals.
The contract covers seven London stations (King's Cross, Liverpool Street, Paddington, Euston, Victoria, Charing Cross and Waterloo) and six regional stations (Birmingham New Street, Edinburgh Waverley, Glasgow Central, Manchester Piccadilly, Leeds Concourse and Liverpool Lime Street).
Network Rail also owns five further stations and Admedia will be adding additional Network Rail stations to its portfolio later this year.
Admedia specialises in washroom media and holds the contract for motorway service stations such as Welcome Break and Moto as well as bars such as Yates's and shopping mall operators such as Westfield.
Steven Wood, advertising manager at Network Rail, said: "We are always looking for ways to unlock the potential for commercial revenue held in our stations.
"Working in partnership with Admedia, this innovative new advertising contract will reach more people every day through eye-catching and exciting new advertising, generating vital funds for us to invest straight back into the railway."
According to figures from Network Rail and supplied by Admedia the toilets in the 13 stations have 25.3 million visitors every year.
Adam Mills, managing director of Admedia, said: "We already have a lot of knowledge about this audience through our motorway service areas and we’ve long been working with the type of brands that would be ideal for Network Rail.
"We are looking forward to offering our existing and new clients access to this new audience via the rail washroom environment."
Admedia will start selling advertising in the stations from this month.
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