This work presents a view of the history of American railroads in the nineteenth century from a somewhat different perspective. The maturation of the railroad is traced through an exposition of the railroad technology that was developed and applied during the period. Throughout the nineteenth century, a symbiotic relationship existed between railroading and technology, each dependent upon the state and progress of the other to a large degree. A great deal of new technology was created for the railroad, and the railroad, in turn, applied new technology as it became available. Volume four is about bridges and tunnels, and signals. An exposition of the various types of bridges, their foundations, and the materials of which they were made is included. Tunnels and marine railroad operations are treated also. The development of signal systems is an area that has been overlooked or neglected in the general literature but is fully covered here. The text of this volume is accompanied by 145 illustrations and accurate drawings of the equipment and appliances, many of which have not been published before outside of old technical journals.