Least used lines in North America?

Which heavy rail lines in the U.S.A, Canada and maybe some other countries with mainline rail are the least used or least busy on the continent?

Probably the easiest way to figure this out (using public information) for the US would be to download the FRA's complete grade crossing database information (that also includes bridges over/under roads). One of the things that railroads are supposed to include (and keep reasonably current) is a count of the number of trains that use the crossing on a daily basis. Since the data also identifies the route, milepost, and location of the crossing, you can use that to get an estimate of how busy a given route is, and which ones are the most or least busy.