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The MTA's "track geometry car" slides around through the New York City subway, employing "a variety of sensors, measuring systems, and data management [software] to create a profile of the track being inspected."
It's a large-scale, precision optical mechanism, measuring such things as track curvature, alignment, "crosslevel," rail gauge, warp, and more, using GPS, gyroscopes, proximity sensors, now-obsolete analogue measuring wheels, strain gauges, accelerometers, and even a "paint spray system" for marking "the location of a defect on the track once a defect is found." These devices can be used in tandem with "ultrasonic rail-flaw detection vehicles" that use ultrasound "to 'look' inside rail to detect flaws unseen by the human eye. An internal flaw may be caused by a poor casting or metal mixture."
But it's hard not to be captivated by the idea of some blindingly well-lit behemoth vehicle maneuvering around beneath the city at night, all lasers, mirrors, lenses, and prisms—a surreal, moving garden of repurposed photographic equipment and motion-capture technologies from different historical eras—scanning the geometry of the metropolis from below, down to thermal flaws in the very metal it passes over. Surrounded by overlapping holographs of infinite lines and tunnels, like the subway dreaming of itself, this collage of physical instruments circles around and around through the foundation of the world, a two-track mind, a mobile neurology thinking in well-measured bursts of strobe light.
(Thanks to Nicola Twilley for the tip!)