Abstract: A method and apparatus for railroad track surveying for accurately determining track gauge includes a pair of electro-optical sensors which are situated completely above the rail head level. A collimated light source in each sensor projects a narrow light pattern containing a substantial vertical component downwardly against the gauge surface of the rail. An optical system in the sensor collects an image of the pattern and focuses it upwardly along an object axis to a focal plane. A detector in said focal plane is responsive only to that portion of the image which appears at a juncture of the focal plane with an object plane which includes said object axis and extends laterally thereof in directions parallel to the plane of the rail heads and intersects the gauge surface at a point five-eighths of an inch below the running surface when the rail head is at its nominal position.