Double tracks make accidents of this character impossible; but introduce a new possibility, that a derailment from any cause upon one track may obstruct the other track so closely ahead of an opposing train that no warning can be given. [Illustration: Central Switch and Signal Tower.] Where trains become very numerous additional safeguards are added by multiplying telegraph stations at short intervals, and giving them conspicuous signals of semaphore arms and lanterns, until finally the road is divided into a number of so-called "blocks" of a few miles each; and no train is permitted to enter any block until the train preceding has passed out. And in the approaches to some of our great depots, where trains and tracks are multiplied and confused with cross-overs and switching service, all switches are set and all movements controlled by signals from a single central tower. Sometimes, by very expensive and complicated apparatus, it is made mechanically impossible to open a track for the movement of a train without previously locking all openings by which another train might interfere. The illustrations on pages 169, 171, and above will serve to give some general idea of these appliances.[17] [Illustration: Mantua Junction, West Philadelphia, showing a Complex System of Interlacing Tracks.] There remains one other branch of the duties of the master of transportation--the proper daily distribution of cars to every station according to its needs, and the keeping record of their whereabouts. And now that the gauges of all roads are similar, and competition enforces through shipments, roads are practically making common property of each other's cars, and the detail and trouble of keeping record of them become enormous. [Illustration: Interior of a Switch-tower, showing the Operation of Interlocking Switches.] The records are made up from daily reports, by every conductor, of every car, home or foreign, handled in his train, and from every station-agent of all cars in his yard at certain hours. From these returns the car accountant reports to their respective owners all movements of foreign cars and gives the transportation department information where cars are lying. The honesty of each other's reports concerning car movements is generally relied upon by railroads, but "lost car agents" are kept travelling to hunt up estrays, and to watch how the cars of their roads are being handled.