Others have said it before, only UK and Japan have a different scale, but also run on 16.5mm track. The rest of the world is true HO scale.
UK uses OO gauge, which is 1/76 scale on 16.5mm track and Japan uses HOj scale, which is 1/80, also running on 16.5mm track, supposedly representing 3 1/2 ft. gauge - the Japanese "standard gauge" for all trains other than the bullet trains.
Modelin a subway is a bit of a challenge, as there is very little choice of equipment. I can´t remember seeing any affordable, mass-produced subway cars than the old Walthers line and the more recent MTH set.. I also can´t imagine, that a 1/80 scale Japanese subway car looks at home on a nothern American layout.
If you do a Google search, you may find your way to a few kichen counter businesses making more recent cars, but beware - they are not cheap!