Interlocking tower

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Image:Des Plaines interlocking tower.jpg

An interlocking tower is an often towerlike structure or building housing the levers for working railroad switches and trackside signals. Its function is therefore that of a control point for movement, course, direction, and speed of trains.

Originally, the switches were mechanically operated by a fairly cumbersome system of levers and rods; the introduction of electro-pneumatic, electromechanical, and electric systems, which required less human strength, allowed towers to control wider areas of tracks.

These towers were, and commonly still are, known as switch towers or signal towers; the introduction of power-assisted systems also brought a feature, called interlocking, due to which it was impossible for the tower operator to route trains on conflicting courses, so that proper successive movement was insured. The name "interlocking" soon came to denote not only such a system or arrangement of switches and signals, but also the tower containing it and even the plant controlled by the tower.

The advent of Centralized traffic control (CTC) in the late 1920s, and its spread, especially from the 1940s, largely eliminated the need for switch towers and ultimately brought about their demise. Indeed, few towers were built from the 1930s on; by the end of World War II, there were over 4000 active towers in the U.S., but only about 100 of them are currently active, and likely all of them are bound to be eventually abandoned. Only three towers survive in Canada, and all three are in Toronto.

Modern U.S. control centers supervise up to several thousand miles, the largest being Union Pacific's Harriman Dispatch Center in Omaha, Nebraska, which controls over 30,000 miles of railroad.