these are able to measure the height and width of rolling stock, so that cars and loads which won't fit under bridges or through tunnels are stopped. most often they are optical line of sight devices that trigger an alarm when the beam is broken. they are often seen at a bearing temperature scanning locations, they then report out as part of the detectors regular train inspection report. mounted on poles or a bridge structure, the optical line is adjusted for a set height and width, it can then send an alarm when something too big to fit the clearance limits ahead passes its view.