I read Trains magazine, and in articles about a railroad, sometimes they'll talk about how many trains per hour. I don't know where they get that from.
If you just want to know if a line is 'busy' compared to 'slow', there's rail line atlases for the US, and Trains mag puts out maps of some areas.
https://www.trains.com/wp-content/uploads/2020/10/trnm0414.pdfOn this map, the highlighted lines between Chicago and Galesburg will be 'busy' and the rest will be 'slow' by comparison
