The AAR, which is handling the search, has contacted undisclosed agencies with requests for information.
Go-Ahead, which also owns South Eastern trains, the Gatwick Express network and London Midland, is seeking an agency to handle all of its integrated communications, spanning print, outdoor, digital and below the line for the Southern network.
The successful agency will be briefed to produce an integrated campaign that seeks to increase passenger growth on the Southern network, which is currently declining by 2 per cent year on year.
Previously, Go-Ahead has worked with WDMP, which recently produced a campaign for London Midland.
The search for an agency comes shortly after the Go-Ahead group's successful re-application for the franchise of Southern trains. It secured the franchise after bidding £300 million to £400 million more than its rivals NedRailways, a Dutch rail operator, and National Express.