Qapla
Engineer
This may help explain the difference:
In the United States, railroad carriers are designated as Class I, II, or III, according to size criteria set by the Surface Transportation Board in 1992 and adjusted for inflation annually. The annual revenue threshold for Class I rail carriers for 2019 was $504,803,294. According to said classification there are seven US Class I freight railroad companies including two Canadian carriers with subsidiary trackage in the US.
The seven Class I freight carriers are BNSF Railway, Canadian National Railway with its subsidiary Grand Trunk Corporation, Canadian Pacific Railway with its subsidiary Soo Line Corporation, CSX Transportation, Kansas City Southern Railway, Norfolk Southern Railway, and Union Pacific Railroad.
In addition, the national passenger railroads in the US and Canada—Amtrak and Via Rail— would both qualify as Class I if they were freight carriers. Similarly, Mexico would have two Class I railroads, Ferromex and Kansas City Southern de México.
A Class II railroad in the United States hauls freight and is mid-sized in terms of operating revenue. Switching and terminal railroads are excluded from Class II status.
Railroads considered by the Association of American Railroads as "Regional Railroads" are typically Class II. An example of a Class II would be the Florida East Coast Railway.