main signals semiphore basics

The first railway semaphore signal was erected in 1841 at New Cross Gate by Charles H Gregory, Engineer of the London & Croydon Railway. During daylight hours, a semaphore signal conveys its indications by a pivoted 'arm'. The arm projects to the left of the post when viewed in the applicable direction of travel, and its angle determines the indication. The first semaphores were three-position signals, used in conjunction with the time interval system. The 'danger' indication was given by the arm being in the horizontal position [2.1]. When in this position, a red light was shown at night. For the 'caution' indication, the arm was lowered through 45°, and a green light was displayed at night [2.2]. In the 'clear' position, the arm dropped down vertically to be hidden inside a slot in the post, and a white light was shown at night [2.3]. By the 1860s, semaphore signals were in widespread use, apart from on the Great Western Railway and the London & South Western Railway. The GWR began installing semaphores in 1865.

Semaphore signal UK

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The first British railway semaphore signals had arms that could be worked to three positions, in the lower quadrant. Used in conjunction with the time-interval system, the arm horizontal meant "danger", inclined downwards at 45 degrees meant "caution" and the arm vertical (arm hidden within the post) meant "clear".