The UK-listed company defeated competition from all the major outdoor contractors before beating JCDecaux in a head-to-head pitch.
The appointment cements Maiden's position as the largest supplier in the 48-sheet market - it now has 40 per cent of sites - and it becomes the second-largest player in the 90-sheet sector, with 29 per cent of sites.
Before the pitch, most of the Network Rail business was shared between Maiden and JCDecaux, which handled 90 per cent between them, with the remainder sold by other smaller contractors.
Maiden has won the contract for an extended ten-year period and has increased its supply of sites by 1,500 48-sheets and 100 96-sheets, distributed nationwide.
David Pugh, the managing director of Maiden Outdoor, said: "This is part of Maiden's plan to rejuvenate the billboard sector - the pendulum has swung too far to roadside six-sheets. We're going to do this by reminding people what great value billboards are, they are a huge creative canvas and, through Postar, they are accountable."
He added that £5 million had now been earmarked to invest in the Network Rail estate through a Maiden rights issue earlier this year. This money will be invested in new packages and products.