line speed

Line speed is the maximum velocity at which any train is permitted to operate on a specific stretch of track, set by the railroad operator and enforced through signaling, cab display systems, or both. It is not the train's capability, but the track's limit. A locomotive capable of 80 miles per hour may be restricted to 45 mph on a section with tight curves, poor rail condition, or dense traffic. The speed is posted in the track operating rules and recorded in dispatch systems.

Line speed varies by section and depends on track geometry, rail weight, ballast condition, bridge load ratings, and signal spacing. Freight lines carrying heavy tonnage often operate at 50 to 70 mph; passenger main lines at 79 to 110 mph depending on track class and investment level. Branch lines, industrial sidings, and approaches to yards typically drop to 20 to 45 mph. Curves impose the sharpest restrictions: a 10-degree curve might limit speed to 35 mph, while a 1-degree curve allows 60 mph.

Enforcement and variation

Modern railroads enforce line speed through automatic train protection (ATP) systems that prevent exceeding the posted limit, or through cab signal displays showing the current maximum. Older lines rely on speed restrictions posted in written rules and driver knowledge. Speed can change seasonally: winter ice may lower line speed on grades, and track work often imposes temporary restrictions that last weeks or months.

Line speed is distinct from track class, which is a design specification, and from operating speed, which may be slower due to train consist weight, weather, or traffic. A train's actual speed is set by the train crew within the line speed ceiling, responding to signals, grades, and operational needs. Exceeding line speed can cause derailment on curves or damage to track; it is a violation tracked by safety systems and cited in accident investigations.