CE 515 Railroad Engineering Railway Track Design Source: AREMA Ch. 6 Stationing& Horizontal Alignments “Transportation exists to conquer space and time -”

Railway Track Design Key factors for rail alignment Operator has no control over horizontal movement Less relative power available for locomotion relative to the mass to be moved Long and thin physical dimension

Railway Track Design Considerations for designing rail alignments Type of train traffic Volume of traffic Speed

Stationing Stationing A single station: 100 feet long from beginning point to end point Milepost Less precise, but more easily identified

Horizontal Alignments Two components: Tangents and Curves Tangents: established first A straight line between two points Denoted with bearings Need a starting point and length to locate in space Curves: established based on tangents Point of Intersection (PI) - fixed Define the tangents Points of curve defined subsequently

Horizontal Curves Important components of simple circular curve Angle of Deflection (I) Other aspects see figure Source: AREMA

Degree of Curve Source: AREMA

Curves: Chord and Arc Definitions Differences between two definition are slight at small degrees of curves But increase as curves get sharper North America freight railways use chord-defined Highway design, some light rail system and other alignments design use arc-defined

Curves Note: the radius of a four-degree curve is not exactly half of that of a two-degree curve Source: AREMA

Centrifugal Force Can be counteracted by superelevation Equilibrium elevation, allow more uniform speed Source: AREMA

Horizontal Alignments Under-balance Difference between the equilibrium elevation and the actual superelevation Limited to three inches by FRA Source: AREMA pg.224

Spiral Curves Transition from tangent level track to curved superelevated track Two functions Gradual introduction of superelevation Guiding the train from tangent track to curved track

Types of Spirals Commonly used: the Searles, the Talbot, and the AREMA 10-chord spiral Two Design Criteria Rotational acceleration of the train about its longitudinal axis Limiting value of twist along car body Recommended formula for the minimum length of spiral on AREMA pg. 225

Questions?