The site http://www.old-maps.co.uk has large scale OS maps going back more than a century for what appears to be the entire UK. The OS 1:2500 series are the ones to go for as these show track plans, mileposts, signal posts and benchmark heights. I turn these into terrain decals for route building.
You can order A4 prints which in 1:2500 scale will get you a mid-sized station. However a month's subscription costs less and gets you access to everything. There's no facility to print but nothing to stop screengrabs.