Graphic, Geometric & Alphanumeric Signals
A special category of all-lighted signals are those that replace or supplement color and/ or position signals with symbols: a graphic design, a geometric form, or letter/number representations. There is even more individual and local practice with these signals than with more conventional all-color forms. Some systems may not have any of these forms of signals and yet other systems have only limited numbers. There is great diversity, nonetheless, in the uses that these signals carry out. This segment will have, if not marked brevity, at least a very provisional character.
Alphanumeric signals are of three basic forms: the mechanical, the multi-lamp, and the stencil (see UK K&W, NSW, QR, Westinghouse Brake and Signal, etc.). The mechanical (also known as the moving slide indicator) is an older form and associated with the semaphore signal. The letter or number to be displayed is moved into position by a moving slide, hence its name. The signal is of large proportions in comparison with other signals. It is employed as a route indicator.
The multi-lamp (or theater-type) consists of a large square signal housing containing the letters of the alphabet and many numbers (Westinghouse В and S). It provides a great measure of flexibility for preparing messages. The signal is so designed that a surface sheet of obscure glass prevents viewing of letters or numbers when a desired message is not lighted up. The stencil has two forms. The larger consists of a rectangular shaped case with up to three symbols. The surface of the case is covered with obscure glass so the letter and/or numbers can not be seen unless the lamps behind the symbols are on. In a variety of cases there are two lamps for each case. There can be a series of cases in a horizontal bank or cases can be attached in a vertical manner. This form of the stencil is for route indications. A second form of stencil is frequently based on standard signal lamps and housing; this form usually has one symbol. It is used for various functions and is often found with a mainline signal. A variety of systems, often outside of Europe, use this form to denote signals that are automatic.
Graphic and geometric signals are usually found together in codes even if some are fully-lighted and others are only partially so. This is reasonable because of the common functions they perform. But in this study fully and partially-lighted signals are divided into different groups even though this results in a split treatment. The compiler has not lost sight of the commonality of such signals.
Dividing signals according to nature of message is an uncertain matter since all dimensions of graphic and geometric signals are virtually fused together. But some tentative remarks on this topic are possible. All revolving signals, whether graphic or geometric, are less than fully-lighted. Of those that are single-sided a limited number are fully-lighted. Most that are all-lighted are graphic rather than geometric.
Fully-lighted graphic signals are found in several Australian systems (for example, NSW). The signal known as a double-crossing indicator is found in a variety of European systems (DB, RENFE being two examples) and is fully-lighted.
All of these involve arrows and some are related to color-light signals.