Railroad classes are the system by which freight railroads are designated in the United States. Railroads are assigned to Class I, II or III according to annual revenue criteria originally set by the Surface Transportation Board in 1992. With annual adjustments for inflation, the 2019 thresholds were US$504,803,294 for Class I carriers and US$40,384,263 for Class II carriers. (Smaller carriers were Class III by default.)
There are six Class I freight railroad companies in the United States: BNSF Railway, CSX Transportation, Canadian National Railway, CPKC, Norfolk Southern Railway, and Union Pacific Railroad. Canadian National also operates in Canada and CPKC operates in Canada and Mexico.
In addition, the national passenger railroad in the United States, Amtrak, would qualify as Class I if it were a freight carrier, as would Canada's Via Rail passenger service. Mexico's Ferromex freight railroad would also qualify as Class I, but it does not operate within the United States.