To install custom maps from smrsimple you must:
- Download the map
- Delete the contents of your “My Games\Sid Meier’s Railroads!\UserMaps” folder
- Extract the zip’d map into the “My Games\Sid Meier’s Railroads!\UserMaps” folder
- Launch Sid Meier’s Railroads! and select the new map.
NOTE: The “My Games\Sid Meier’s Railroads!\Custom Assets” folder must also be empty. Some exe installer based maps use this folder and can interfere with new maps.>
Mac OSX users will want to read the alternative guide: http://smrsimple.com/2012/11/custom-maps-smr-mac/