It is understood this is a new brief following Trainline's acquisition by US private equity firm KKR in January.
Bite said its remit covers corporate and consumer PR and social media activation across Europe: "Bite will work alongside the new Trainline management team and other rostered agencies to deliver integrated, creative campaigns that demonstrate how rail passengers can save time, money and hassle."
The agency selection process was handled by AAR.
Simon Darling, commercial director at Trainline, said: "We’ve set ourselves a tough brief. We’re on a mission to break a 200-year-old habit and get Britain getting its train information and tickets via our app rather than staring at platform indicator boards and standing at ticket machines.
"Bite showed itself to be up to this consumer PR and social media challenge. And it is leading the charge on us getting our head above the parapet and being recognised as a technology industry leader, a talent leader and a rail industry thought leader."
Bite MD Kath Easthope said: "The Trainline team has big plans for the company’s future and we love how integrated we’ll be with its other agencies."
KKR acquired Trainline from Exponent in January, which derailed plans to float the business on the stock market with a reported value of around £500m.
FTI Consulting represented Exponent for corporate comms over the period of the sale and has not been involved with Trainline since.
Fishburn has also worked with Trainline and is currently listed as its PR contact. Fishburn was unable to comment on its involvement in the Trainline account at the time of publication.