Railroads are classified based on their annual operating revenues.
To fall under the railroad classes they must have tracking in the USA, which is why Canadian National and Canadian Pacific are Class I and Ferromex is not.
The class to which a carrier belongs is determined in accordance with the following revenue thresholds:
Class I railroads are regulated by the Surface Transportation Board (STB).
The revenue thresholds are periodically updated to account for the impact of inflation.
The revenue numbers were last updated in 2017.