It is the purpose of this book to collect from various sources that which is already in use in common practice in the field of railway signaling and to present it in text-book form suitable for the beginner in his study of this subject. Much of the descriptive material and many of the drawings were furnished by the various signal and supply companies specially for this book. Other descriptions and drawings were taken from catalogues and descriptive literature issued by these companies. I have not included any thing concerning specifications for the construction, installation and maintenance of materials. This is a voluminous subject in itself; besides, specifications for practically every item of equipment that enters into railway signaling are provided for in the Manual of the American Railway Association, Signal Section.<br><br>In a few cases, I have quoted from the Proceedings of the American Railway Engineering Association, from the Signal Dictionary and from the Railway Signal Engineer. As I have drawn so largely from the Proceedings of the Railway Signal Association, it might be pertinent to state briefly that in its early days the organization was known as the Railway Signaling Club. Later it changed its name to the Railway Signal Association; and recently during the time when the railways were under the supervision of the Director General of Railroads, United States Railroad Administration, the organization amalgamated with the American Railway Association and took the name which it still retains, the American Railway Association, Signal Section. I might state in this connection, also, that the Manual and all the Proceedings of the organization under both the old and new regimes may be obtained from the Secretary, Mr. H. S. Balliet, 75 Church St., New York.<br><br>I want to express my appreciation for the help received from all sources, for the material furnished, for the suggestions offered and for the corrections made in the preparation of the manuscript.