Any track gauge narrower than the standard 4 feet 8.5 inches. Railroads built to these dimensions use smaller, lighter equipment and can navigate much tighter curves than standard equipment. This made them highly economical for rugged mountain terrain, logging operations, and mining routes where blasting standard-width cuts and tunnels was too expensive.
Heard on the job
The old mining company ran a three-foot narrow gauge line right up to the tipple.
Insider tip
The most common narrow gauge in North America was 3 feet, famously used by the Denver and Rio Grande Western in the Rocky Mountains.