The vast majority of North American railroads are standard gauge (4 ft 8+1⁄2 in / 1,435 mm).
As well as the usual reasons for having one gauge i.e. being able to operate through trains without transfer arrangements, the North American continent-wide system of freight car interchange with rolling stock having the same standard gauge, couplings, and air brakes meant that individual companies could minimise their rolling stock requirements by borrowing from other companies.
See Rail transport in Mexico The Panama Railroad was originally 5 ft (1,524 mm) as in much of the Southern United States.
Nowadays only the ship handling trains along the Panama Canal, called mules, still have the 150 cm / 5 ft track.
The final conversion to true standard gauge took place gradually as track was maintained.