Excerpt from The Railway Signal Dictionary
Railway signaling is a comparatively new art, and a rapidly growing one, so that its nomenclature is unsettled and its practices varied; and numerous changes of a somewhat radical nature are still going on. The development of scientific signaling, as distinguished from the earlier and empirical methods of controlling the movements of railway trains, has proceeded from a curious combination of motives; in some cases, from the enterprise of trained men, in others from a pressing necessity due to expansion of business, and in still others to the force of public opinion, both the public and the railway officer being sometimes actuated by injudicious haste. That we are not yet out of the transition state is no cause for surprise.
In making a signaling dictionary the very first step calls for arbitrary action. Hand-motion signaling, with or without the use of flags and lanterns, is excluded, because those features have no need of the services of the signal engineer. A "slow-board" is a fixed signal, but the signal engineer scarcely gives it a thought. On the other hand, automatic bells at highway crossings logically do not come under the head of railway signals, for they do not directly affect the movement of trains; but the signal engineer does take cognizance of this class, because it requires skilled attention and is usually put in his charge.
In fixed signals, which constitute the subject of this work, American railway officers often make a distinction which is wholly arbitrary and, in most cases, useless - that between interlocking signals and block signals. The function of a signal being clearly stated, the English nomenclature, ignoring this distinction, is entirely adequate. But to make each collection of drawings complete in itself, we recognize this distinction; and the reader will find under the two different heads - different illustrations showing semaphore signals exactly alike.
Other uses of words are inexact, yet not troublesome. The noun "signal" usually means a semaphore signal complete, yet in speaking of the movements of a signal, especially where there are two arms on a single post, "signal" refers to the movable part alone. "Controlled manual" means signals worked by hand but having the hand levers fitted with electric locks by which the man working the signals is subject to the control, under certain circumstances, of the signalman at another station, and also to control by the train itself through track circuits connected with the lever locks; but there are now at least three principal divisions of this subject, so that the simple term "controlled manual" has little meaning until it is explained. It includes: (1) A system worked without track circuits or any other means of control by trains; (2) one worked with a short track circuit at each block station, and (3) with a track circuit throughout the length of the block section.
Practice is more varied than nomenclature; but it is not so inconsistent as it seems, for with suitable limitations of the speeds of trains, supplemented by careful instruction of enginemen, the extreme requirements of the uniformity enthusiasts are not absolutely essential. For example, thousands of miles of railway equipped with home block signals have very few distant signals, and this necessitates slackening of speed in snowstorms and fogs. Many block stations on single-track lines have but one signal for each direction, and both of these are supported on a single post, opposite the station office; but with a rule requiring trains to approach stations with speed under control, this arrangement is workable. On many roads this and other deficiencies of the block system are overcome by continuing in force the time-table regulations. The block system is a valuable means of safety even when worked under these modifications.
Semaphores have wholly superseded disks for signaling at yards and terminals, thi. Это и многое другое вы найдете в книге The Railway Signal Dictionary (Classic Reprint)