This illuminated map sets upon the best piece of mechanism that has yet been devised for the operation of the terminal yard. It is a long boxed affair, not entirely unlike the box of the old-fashioned square piano, but in this case (the terminal we are watching being of unusual capacity) more than thirty feet in length. This box is the very brains of the terminal. It represents the acme of mechanical condensation. Reduced to its earliest and simplest equivalent--the separate hand operation of a gigantic cluster of switches in a great terminal yard--it would cover a vast area and result in the employment of an army of switchmen. Carelessness on the part of any one member of this army might cause a serious accident. The margin of safety would be very low in such a case. The first schemes of automatic switch systems eliminated the necessity of employing an army of switchmen. A cluster of levers, in a tower of commanding location, was connected by steel rods with the switches and the signals which protected them. A man in the tower operated this group of levers. In this way, the control of the yard was simplified, and responsibility was placed upon a better paid and better trained man than the average hand switchman. The margin of safety was considerably broadened. Then came an amendment to that first system. Some genius of a mechanic built an interlocking switch machine, a thing of cogs and clutches, by which a collision in a railroad yard became almost a physical impossibility. In these mechanical interlocking devices the tower levers are so controlled, one by another, that signals cannot be given for trains to proceed until all switches in the route governed are first properly set and locked; and conversely, so that the switches of a route governed by signal cannot be moved during the display of a signal giving the right of way over them. By installation of the interlocking, some of the responsibility is taken by mechanical device from human brain and the margin of safety broadened still further. This "piano box" represents still further condensation of the switch and signal control and interlocking devices. The men who designed this particular city gate designed it to accommodate more than a thousand outgoing and incoming passenger trains each twenty-four hours; they had found that the condensations given by earlier systems were not sufficient for their purpose. After bringing several switches, designed to act in concert, upon a single lever, they found that they would have a row of 360 levers. Set closely together these would require a tower about 160 feet long. It is roughly figured that it is not desirable to assign more than twenty of these heavy levers to a single towerman and that meant eighteen men, working at a shift. Moreover, the throwing of a heavy switch half a mile distant from the tower is not a slight manual exercise.