Produced by the Union Switch and Signal Company, these bulletins are a valuable resource for railway signal engineers and enthusiasts interested in the history of railway signaling. Covering a wide variety of topics, from interlocking machines to switch locks, this series is recommended for those interested in historical railway technology. This work has been selected by scholars as being culturally important, and is part of the knowledge base of civilization as we know it. This work is in the "public domain in the United States of America, and possibly other nations. Within the United States, you may freely copy and distribute this work, as no entity (individual or corporate) has a copyright on the body of the work. Scholars believe, and we concur, that this work is important enough to be preserved, reproduced, and made generally available to the public. We appreciate your support of the preservation process, and thank you for being an important part of keeping this knowledge alive and relevant.