The distance between the inner faces of the two running rails of a railroad track. North American and most world railroads use standard gauge of 4 feet 8.5 inches (1,435 mm). Gauge determines equipment compatibility and is a fundamental physical standard of the railroad network.
The rail gauge of 4 feet 8.5 inches (1,435 mm), used by approximately 55% of the world's railways including all Class I railroads in North America. Standard gauge enables interoperability of equipment across the North American network without gauge conversion. It was adopted as the U.S. standard in the 1880s.
The weight of rail measured in pounds per yard, which determines its load-carrying capacity, stiffness, and durability. Mainline tracks typically use 132 to 141 lb/yd rail while branch lines may use lighter 90 to 115 lb/yd sections. Heavier rail supports higher axle loads and train speeds.
The transverse member placed beneath and perpendicular to the rails to maintain track gauge and transfer load to the ballast and subgrade. Traditional ties are wood (often treated with creosote), while modern alternatives include concrete, composite, and steel. Tie replacement is one of the largest track maintenance cost items.
The granular material, typically crushed stone or gravel, placed under and around ties to provide a stable, well-drained foundation for the track structure. Ballast distributes axle loads to the subgrade, provides drainage, and resists lateral and longitudinal movement of the track. Ballast maintenance and tamping are routine track maintenance activities.