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.