A handful of agencies have already been contacted to pitch for the retainer, with a decision expected later this month.
Network Rail's lobbying is overseen by head of public affairs Ed Wilson. The rail operator has used MHP, previously Mandate Communications, on a retained basis since 2007. It has also used Edelman for public affairs projects in recent years.
A spokesman for Network Rail said MHP and Edelman had been invited to repitch.
Last week, the Office of Rail Regulation unveiled plans to make Network Rail justify bonuses paid to bosses.