https://www.trains.com/trn/train-basics/abcs-of-railroading/track-classifications/

Track classifications With about 177,200 miles of track in service as part of the interstate railroad system, the railroads and the FRA have their hands full monitoring the system’s condition. As part of its jurisdiction, FRA categorizes all track in six classes, segregated by maximum speed limits. They are:

Class 1: 10 mph for freight, 15 mph for passenger. Much yard, branch line, short line, and industrial spur trackage falls into category.

Class 2: 25 mph for freight, 30 mph for passenger. Branch lines, secondary main lines, many regional railroads, and some tourist operations frequently fall into this class. Examples are Burlington Northern Santa Fe’s branch from Sioux Falls to Madison, S. Dak.; Napa Valley Wine Train’s 18-mile ex-SP line between Napa and St. Helena, Calif.; and the entire Strasburg Rail Road, 4 and-a-half miles between Strasburg and Leaman Place, Pa.

Class 3: 40 mph for freight, 60 mph for passenger. This commonly includes regional railroads and Class 1 secondary main lines. Examples are BNSF between Spokane and Kettle Falls, Wash.; and Canadian National’s Wisconsin Central line between Neenah, Wis., and Sault Ste. Marie, Mich.

Class 4: 60 mph for freight, 80 mph for passenger. This is the dominant class for main-line track used in passenger and long-haul freight service. Examples are most of the suburban trackage of Chicago’s Metra commuter railroad, including its own Rock Island District west of Blue Island and Milwaukee District West Line west of Bensenville (also a Soo Line freight route); plus BNSF west of Cicero Yard and Union Pacific (former C&NW) west of Proviso Yard; New England Central’s entire main line between New London, Conn., and East Alburgh, Vt.; and the Arizona & California (ex-Santa Fe) between Matthie, Ariz., and Cadiz, Calif.

Class 5: 80 mph for freight, 90 mph for passenger. This is the standard for most high-speed track in the U.S. Examples are UP’s main line between Council Bluffs, Iowa, and North Platte, Neb.; and BNSF between Fullerton and San Diego, Calif., used mostly by Amtrak’s Pacific Surfliner trains to San Diego.

Class 6: 110 mph for freight, 110 mph for passenger. This is found in the U.S. exclusively on Amtrak’s Northeast Corridor between New York and Washington, D.C. Amtrak has also received special “Class 7” status for 125 mph operation and (with the launch of high-speed Acela Express trains) “Class 8” status for 150 mph on specific segments of the corridor.

Excepted track. In addition to the six numbered classes, FRA track standards also provide for something called “excepted” track, which carries a 10 mph speed limit for freight but cannot be used by revenue passenger trains. FRA permits excepted track under very narrowly defined conditions.