The invitations for tender were issued on Monday and the five-year contract has the option of being extended for a further two years, dependent on the winning contractor's performance.
The railside contract includes all rail-facing ad sites across the Network Rail estate, including six-sheets, Transvision screens and large-format sites across the 18 Network Rail-managed stations.
On 18 January JCDecaux bought the assets of Titan Outdoor out of administration, including the Network Rail contract.
Dominic Pendry, the head of communications for property at Network Rail, said: "Network Rail is looking for a partner to work with in the long term.
"We are looking to invest in the estate and improve the outdoor media over the coming years."
The Network Rail roadside contract is also currently up for tender.