To install custom maps from smrsimple you must:

  1. Download the map
  2. Delete the contents of your “My Games\Sid Meier’s Railroads!\UserMaps” folder
  3. Extract the zip’d map into the “My Games\Sid Meier’s Railroads!\UserMaps” folder
  4. Launch Sid Meier’s Railroads! and select the new map.
You can only have one custom map installed at a time.

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/