The news comes as Viacom, Clear Channel and JCDecaux weigh up bids for Maiden, after the outdoor contractor revealed at the end of last year that it had piled up debts of £40 million.
Viacom has been appointed by FirstGroup, which was given the task of operating two newly created rail franchises at the end of 2005.
The First Great Western business, an amalgamation of First Great Western, First Great Western Link and Wessex Trains, includes standard sites at all stations.
The First Capital Connect franchise covers all sites across the current Thameslink and WAGN Great Northern franchises.
The deal will see Viacom's share of the UK rail market rise to almost one-third.
Jason Cotterrell, the national rail franchise director at Viacom Outdoor, said: "We see rail increasing its share of advertising budget over the next few years."