OpenRailMap is good all around.
- For Canada, check out the Canadian Railway Atlas. Works pretty fast, eh.
- For USA Class I Freight rail, check out this map. (Note it is very slow to use). USA national passenger rail Amtrak's system map/trip planner is here
- For Mexico, check out the Mexican Government ARTF's Sistema Ferroviario Mexicano interactive map (Also very slow). On the flip side they offer a direct download to their map files as well.