Upper quadrant signal

One of the earliest forms of fixed railway signal is the semaphore. These signals display their different indications to train drivers by changing the angle of inclination of a pivoted 'arm'. Semaphore signals were patented in the early 1840s by Joseph James Stevens, and soon became the most widely used form of mechanical signal. Designs have altered over the intervening years, and colour light signals have replaced semaphore signals in most countries, but in few they remain in use. Note that this article doesn't represent a worldwide view of the semaphore.

The first railway semaphore signal was erected by Charles Hutton Gregory on the London and Croydon Railway (later the London Brighton and South Coast Railway) at New Cross, southeast London, about 1842 on the newly enlarged layout also accommodating the South Eastern Railway.John Urpeth Rastrick claimed to have suggested the idea to Hutton Gregory.

The semaphore was afterwards rapidly adopted as a fixed signal throughout Britain, superseding all other types in most uses by 1870. Such signals were widely adopted in the U.S. after 1908.

The semaphore arm consists of two parts: A wooden or metal arm (or "blade") which pivots at different angles, and a spectacle holding coloured lenses which move in front of a lamp in order to provide indications at night. Usually these were combined into a single frame, though in some types (e.g. "somersault" signals in which the arm pivoted in the centre), the arm was separate from the spectacle. The arm projects horizontally in its most restrictive aspect; other angles indicate less restrictive aspects.

The appropriately coloured lens, depending on the arm's position, is illuminated from behind by either an oil lamp, a gas lamp, or an incandescent lamp run at a low voltage (white LED clusters have also been tested for this purpose). Where a green light was required, a blue lens would usually be used. When combined with the mainly yellow-emitting flame of an oil lamp, this produced a green colour; it was important that the resultant colour was not even yellow-green in appearance, as this could have been confused for a distant signal at caution. Later signals using electric lamps used green lenses. Some signals converted to electric lamps from oil used a yellow-tinted bulb with the original blue lens to maintain the correct colour.

...
Wikipedia