railroad track

A railroad track consists of two parallel steel rails fastened to a series of wooden or concrete sleepers (called ties in North America), which are themselves laid on a prepared roadbed or ballast. The rails are typically rolled steel sections with an inverted T-shape in profile, with a flat base, vertical web, and rounded head. The head provides a running surface for the wheel flanges, while the base distributes load into the ties. Standard gauge in North America is 4 feet 8.5 inches between the inner edges of the rails; other countries use different gauges, notably 1,435 mm (effectively the same) in Europe and 1,000 mm in some mountain and industrial railways.

The rails themselves come in various weights and profiles depending on service demands. Heavy-haul freight lines use rail sections of 130 to 155 pounds per yard; passenger and lighter industrial lines may use 100 to 115 pounds per yard. Modern continuously welded rail, laid as single pieces several miles long, has largely replaced jointed rail with bolted fish-plates, which reduces maintenance and wheel impact at joints. The rails are held in gauge (the correct spacing) by tie rods or lateral bracing and are typically secured to ties with spikes, clips, or fastening systems designed to allow some lateral movement while preventing creep, the gradual shifting of rails under repeated stress.

Ballast and the foundation

The ballast, usually crushed stone or gravel, sits beneath the ties and serves multiple functions: it distributes the concentrated load from ties over a wider area of roadbed, allows drainage away from the track, and provides lateral restraint. On poor drainage routes or in wet climates, the ballast depth may reach 12 to 15 inches; arid regions may use 8 to 10 inches. The roadbed itself must be properly graded and compacted to prevent settlement; differential subsidence causes wavy track and poor wheel contact, leading to accelerated wear and derailment risk.

Wooden ties remain common in North America and are typically 7 by 9 inches in cross-section and 8.5 feet long, spaced 19 to 21 inches apart on tangent track and closer on curves. Concrete ties, heavier and longer-lasting but less forgiving of misalignment, have gained ground in heavily trafficked corridors and in regions with severe weather or wood-damaging pests. Wood ties absorb moisture, rot, and must be treated with preservatives; concrete ties are immune to these problems but cannot be driven with spikes, requiring special fastening clips.

Track geometry is critical: rail must be properly crowned on curves (the outer rail raised relative to the inner), typically 4 to 6 inches of elevation on main-line curves, to balance centrifugal force and normal weight. Rails wear at different rates depending on traffic tonnage, curvature, and speed; inner rails in tight curves wear faster. Regular inspection for rail defects (internal flaws detected by ultrasonic testing), corroded fasteners, and tie degradation is essential; neglected track accumulates delays and accident risk. The entire system is a balance between stiffness (to support heavy loads and train dynamics) and compliance (to distribute impact and allow drainage).