O Gauge is a standard size for model trains, traditionally built to a scale of 1:48 in North America. The gauge refers to the 1.25-inch distance between the outer rails. While strictly a hobbyist term, real railroaders often encounter it through the massive cultural crossover between the industry and scale modeling.
Heard on the job
He retired off the road last year and spends his time building an O Gauge layout in the garage.