The STB divides railroads into three classes based on their annual operating revenues:
- Class I – $447,621,226 or more
- Class II – $35,809,698.01 to $447,621,225.99
- Class III – $35,809,698 or less
These revenue thresholds are periodically updated to account for the impact of inflation. The numbers above were updated in 2017 and were still active at time of this writing in 2019.
If a railroad sees three consecutive years of operating revenue within another category, the STB will reclassify that carrier effective January 1 following the determination.