Counting only public (ie not preserved/tourist/heritage lines) railroads but including subway and trolley systems how many different track guages are there in the US?
Here in Europe I can think of plenty, 750mm, 891 mm (3ft), 1 metre, 4 ft, standard, 5 ft, 5ft 3 and 5ft 6. Plus one or two strange trolley guages. Apologies for the mixed measurement units, and minor kudos for those who can name where to find the various guages.
I may be mistaken but I think there are only two:
4’ 8_1/2" Standard Gauge
5’ Bay Area Rapid Transit (San Francisco)
U.S. Gypsum in Plaster City, CA operates a 3 foot gauge industrial railroad through the California desert.
Also: 3’, US Gypsum at Plaster City CA and various steel mill operations
4’ 10.5", SEPTA light rail, ex-Philadelphia & West Chester
BART is actually 5’ 6"
Bay Area Rapid Transit and United Gypsum have already been mentioned. Philadelphia has some rapid transit lines built to 5’ 2.5", a hold-over from when cities mandated incompatibility to keep trolley lines from using their tracks to haul freight cars. (Baltimore had 5’ 4", but all existing transit trackage is standard gauge.)
3’ gauge: United Gypsum has been mentioned, and technically the White Pass & Yukon out of Skagway, Alaska is in a gray area between heritage and commercial, as is the Durango & Silverton.
Aside from that, I believe the only place you’ll find other gauges are excursion/heritage operations, a few scattered oddball in-plant industrial applications (there was a brickyard using 2’ gauge, and a tie-making plant using 3’ gauge, and a steel plant using a ten-foot-gauge overhead trestle line for scrap metal transfer), and the East Erie Commercial RR at the GE plant in Erie, Pa. which needs various gauges to move and test export locomotives (meter gauge, 5’ 6", etc.).
San Fransisco: BARTD wide gauge as noted.
MUNI, standard gauge
Cable cars, about 4ft-2in.
Pittsburgh is 1/4-inch wider or narrower than Philadelphia. Philadelphia Light Rail gauge is also on Market - Frankfort third rail elevated-subway line and local streetcar system.
Toronto streetcar, light rail, and third-rail rapid transit about 4ft-10-1/2 inches.