Gilmartin will remain involved as a senior advisor, supporting the management team and Trainline's wider industry partners in driving industry digitisation and long-term growth, the group said.
Chief operating officer Jody Ford, who joined the company last month for this newly created role, has been appointed as Gilmartin's successor.
He was previously chief executive at photo printing and gift personalisation site Photobox Group, encompassing the Moonpig and Photobox brands, and held senior roles at eBay Inc.
“The decision to step down next year is a personal one; after seven years at the helm the time has come for me to spend more time with my family,” Gilmartin said in a statement.
Shares slipped 6% to 311.6p early on Tuesday.
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