To start off, this one from KCS,,a general map of modern lines
http://www.kcsouthern.com/en-us/KCS/Documents/system_map[1].pdf
I just tested the above link, it looks like you will have to manually enter the [1].pdf part
Pere Marquette Maps
PM system
http://www.pmhistsoc.org/maxpmmap.shtml
PM still in use
http://www.pmhistsoc.org/curpmmap.shtml
Detroit & Mackinac
http://www.michiganrailroads.com/dmhs/Programs/Map.htm
http://www.michiganrailroads.com/RRHX/Railroads/D&M/D&MSystemMap1922.htm
many states have railroad maps chowing all of the railroad in that state.
New York Central
http://www.american-rails-forums.com/AR%20Images/Maps/new-york-central-railroad-map.jpg
Penn Central
http://www.american-rails-forums.com/AR%20Images/Maps/penn-central-railroad-map.jpg
Rock Island
http://www.american-rails-forums.com/AR%20Images/Maps/rock-island-railroad-map.jpg
Nickel Plate Road
http://www.american-rails-forums.com/AR%20Images/Maps/nickel-plate-road-system-map.jpg