The agency will be looking to help increase market share, as well as raising awareness of services and destinations along the routes.
Activity will be led by Grayling's public services team, which has previously worked with Wrexham & Shropshire railways.
Work for CrossCountry will be delivered across the UK, with Grand Central working with teams from the agency in both London and the North.
Louise Blyth, head of marketing, CrossCountry, said: ‘Effective communication is critical to our success and we feel assured that Grayling will drive a connection with our customers.'
She added that the agency’s regional network was also key to the delivery of good comms for the rail businesses.
Zoe Dubber, associate director, Grayling Public Services, said: ‘We will use our expertise and insight to deliver creative, yet realistic, campaigns to help convey the many benefits of train travel. During the next year or so, we’ll be focusing on targeting families, time-rich older people, students and business travellers, to name but a few.’