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After World War I, the task of handling individual cars and making up trains was facilitated by the automation of the classification yards where these tasks were handled. One of the first yards of this more modern type was the Markham Yard of the Illinois Central Railroad, just south of Chicago. The factor whose development allowed the introduction of large-scale automation was the remote-controlled retarder used in concert with power-operated switches and a hump to enable all the yard’s activities to be controlled from a central tower with good all-round fields of vision. In the first period of North American railroad operations, the movement of all the cars in a yard was undertaken by individual switching of the engines to move them into the siding whore their train was being made up. This was a slow and laborious system that was greatly simplified in the 1880s by the introduction of humps, or artificial mounds, over which all the cars could be pushed in turn and then coasted downhill, with the appropriate switch setting, to their allotted sidings. The system was further speeded by the introduction of power-operated switches, but reached its definitive form with the installation of retarders, which are track- side beams that can be operated to grip the wheels of passing cars and thereby stow them, to allow full control of all the yard’s activities from a central location.
The Markham Yard opened in 1926, and at that time the northbound classification section had 121 retarders and 69 pairs of switches feeding 67 tracks, with five towers to supervise operations. The advent of computerized control systems has further enhanced this already improved concept by making possible completely centralized control. Such a facility is CP Rail’s Alyth Yard at Calgary where, with the exception of the uncoupling of the cars as they pass over the hump, the entire operation is controlled by a central computer. Further capability is provided to systems of this tort by the use of automatic car identification, ensuring that the right car is sent to the correct siding: the label marking on the side of each car is read by a photoelectric scanner so that the type of car, its owner and its individual identification can be established with precision. Extended in scope by the location of other photoelectric scanners at key points on the main lines, this is the system that makes it possible for computerized central systems, such as the Southern Pacific Railroad’s TOPS, to ‘know’ the exact position of every car in the system.
The effective use of the retarder is based necessarily on a nice calculation of a number of factors, including the car’s type, whether or not the car is loaded, the car’s degree of freedom in running, the strength and direction of the wind, and the route to the individual siding. The integration of all these factors is necessary for assessment of the amount of pressure each pair of retarders exerts on the wheels to make certain that the car neither ceases to move before reaching the steadily varying numbers of other cars already at a halt in the siding, nor crashes into them.
It was the very number of the variable factors, and the need for human crews to keep the cars under observation right through the shunting process, that demanded the use of several control towers in each major yard, until the advent of the computer made it feasible to automate virtually the whole of the process and thereby permit a reduction in the number of control towers to one. Such automation is expensive in capital terms to establish, but allows significant savings once it has entered service. Manpower can be reduced, and the swifter and more accurate classification of trains makes for much more efficient use of the railroad’s assets; the Alyth Yard, for example, has the space for 5,200 standing cars, and can handle the movement of 3,000 cars every day.