The combined business will allow passengers to book tickets across Europe, providing access to 22 countries and 36 operators across the continent.
“For too long, booking rail tickets in Europe has been an overwhelming and complicated process for customers," said Trainline chief exec Clare Gilmartin.
"We’re hugely excited that, with this acquisition, Trainline is making European rail travel easier and more accessible than ever before."
Meanwhile, Captain Train chief Jean-Daniel Guyot said: "We share the same ambition, and it was a natural step for us to decide to work with Trainline. The complementary nature of our offerings and the scope of the markets that we will cover will enable us to reach our shared objective of making rail travel in Europe easier.”
Trainline was acquired by KKR in 2015 with the aim of accelerating Trainline’s growth in the UK and supporting the company to expand into continental Europe and beyond. Philipp Freise, KKR's head of technology, media and telecoms in Europe , said: “One year ago when KKR first invested in Trainline we had the vision of creating the clear global leader in digital rail mobility.
"The combination of Trainline and Captain Train is an important step on this journey, and will bring together a management team of world class talent in rail, tech, product and marketing.”