The U.S. railroad industry consists mostly of freight railroads but also serves passengers. Freight railroads are divided into classes based on revenue. Class I freight railroads earn the most revenue and generally provide long-haul freight service. Freight railroads operate over approximately 160,000 miles of track and own most of the track in the United States; a notable exception is the Northeast Corridor, between Washington, D.C., and Boston, Massachusetts, which Amtrak predominantly owns. Amtrak provides intercity passenger rail service in 46 states and the District of Columbia and operates on 21,000 miles of track. Commuter railroads serve passengers traveling within large metropolitan areas and most operate over track infrastructure owned by Amtrak or freight railroads for at least some portion of their operations. Specifically, 9 commuter railroads operate over Amtrak-owned infrastructure. Sixteen commuter railroads operate over infrastructure owned by freight railroads.