track geometry of each rail or the track center line onto the horizontal plane," (FRA Definition). Also known as the "straightness" of the tracks. Crosslevel – The variation in cant of the track over the length of a predetermined "chord" length (generally sixty-two feet). On straight or tangent track, ideally there should be no variation, while on curves, a cant is generally desired. Curvature – The amount by which the rail deviates from being straight or tangent. The geometry car checks the actual curvature (in degree of curvature) of a curve versus its design curvature. Overhead lines (or catenary) – Measures the height and stagger of contact wire, the position of catenary masts or poles, and the positions of the wire bridges if applicable. Track gauge – The distance between the rails. Over time, rail may become too wide or too narrow. In North America and most of the world, standard gauge is . Rail profile – Looks for rail wear and deviations from standard profile. Warp – The maximum change in crosslevel over a predetermined chord length (generally sixty-two feet). Track geometry cars used by the New York City Subway also measure: Corrugation of running rail surface Tunnel and station platform clearances Third rail height and gauge Vertical gap between third rail and protective board Non-contact measurement and inspection methods Laser measurement systems – Measures rail profile and wear, crosslevel, and track gauge Accelerometers Used to measure alignment by finding the acceleration in a certain direction and then integrating until a position is obtained. These positions are then used to create artificial chords to measure various parameters. Used to obtain ride quality measurements. If certain accelerations are reached or exceeded freight can be damaged or passengers may become uncomfortable. Video system – Captures video of the right-of-way for further analysis, as well as for machine vision inspections of certain track components Gyroscope – Oriented in the vertical direction, used to measure cross level and warp. These are now obsolete, having been replaced by laser measurement systems. Proximity sensor – Used to measure surfaces, alignment, and gauge. These are now obsolete, having been replaced by laser measurement systems. Contact measurement and inspection methods Measuring wheels – Mostly obsolete, originally used for measuring nearly all parameters, these have now been replaced by lasers Strain gauges – Used in conjunction with the measuring wheels to translate the various movements of the measuring