The GIF animation shows a two-arm lower-quadrant signal. The upper arm is the home arm, and the lower one the distant arm. They have the shapes and colors that were standard both in the US and the UK. The spectacles are the continuous-light type with three roundels each, although the arms have only two positions, horizontal and depressed 60°. At first, the signal shows Stop, red over yellow. The distant arm must be horizontal when the home arm is horizontal; this is guaranteed by the mechanism. The home arm now clears. Note that red is shown until the arm is well depressed. The signal now shows Approach, green over yellow. Finally, the distant arm clears, and eventually shows green. The signal now displays Clear, green over green.
Oil lamps displayed the night aspects. This was usually a long-burning lamp that was lighted continuously. Later, an electric lamp was used, after the electrical supply became reliable. Oil lamps were retained because of their reliability, as well as to save batteries. The arms were operated by motors in the signal base (Union Switch and Signal) or by motors behind the arms on the post (General Railway Signal). They were heavily counterweighted to return to horizontal in case the mechanism broke. At interlockings, the signals were operated by pipes from the tower. In this case, they often had a third, shorter, arm below the two main arms to display a Restricting aspect.
These signals were preferred by the Union Pacific, Southern Pacific, and Chicago, Burlington and Quincy for many years, and performed very reliably until the 1960's and even later in some cases, such as on the Southern Pacific. Of all signals that have ever been used, they were the easiest to see and interpret by daylight. With a sky background, they could be reliably interpreted at a mile or more, as soon as they could be distinguished. They were easier to interpret than the typical three-position upper quadrant semaphore, and were preferred by some signal engineers for this reason. They first appeared as motor-operated signals shortly after 1900, and had assumed this typical appearance by the time of the First World War.
Composed by J. B. Calvert
Last revised 19 May 1999