JCDecaux Airport has won the advertising contracts for the Gatwick Express and the Stansted Express rail services.
The year-long deal covers all internal and external advertising on board all the South East rail-air links managed by marketing company Airport Express, including ambient opportunities, such as train wraps.
Previously, Maiden held the contract to sell advertising on Gatwick Express trains.
The contract marks the first use of advertising on the Stansted Express.
JCDecaux Airport marketing director Richard Malton said: "This is a significant addition to our portfolio and a pleasing development in our relationship with Airport Express.
"This contract provides another channel for advertisers to target the 100 million passengers who travel through BAA's three South East airports and strengthens the opportunity to create advertising touch points throughout the journey."Airport Express commercial manager Graeme Hay said: "We are looking forward to working with JCDecaux to develop ways of using our unique environment for advertisers to reach the elusive traveller."