The regional atlases from Steam Powered Video are excellent.
I own a US one and a Texas one. They are outdated now, but are usually really good.
IMO there are many things that are usually desirable in a rail atlas:
Labels for current and past owner of tracks
tunnels and bridges
differentiation between common standard gauge and electrified
a level of detail that shows every industrial lead or spur.
labels for yards, stations(actual passenger ones and on-paper ones known by rail dispatchers), control points, junctions, anything that's notable really. Both for that kind of railfan who owns a scanner or just people like us who probably read the Railroads.net forums to interpret the rail-speak like "The Glenwood sub between MP 141 past Rose Jct is getting double tracked"
A map of tracks is nice but nothing beats a really well put together collection of information.