Abstract: The invention is a system for measuring the gage of railroad tracks and includes a pair of sensor probes mounted a fixed distance apart on a wheeled vehicle traveling along two rails and each located in positions horizontally adjacent to one of the rails. Each of the probes retains spaced apart first and second collimated light sources that direct first and second light beams at incident angles against the inner surface of the adjacent rail head at points five-eighths of an inch from the top surface thereof. After reflection from the rail head the first and second beams produce images in an optical receiver also retained by the probe. A detector produces an output dependent upon the spacing between the first and second beam image centers which is in turn dependent upon the distance between the probe and the adjacent rail head.