Interlocking tower
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.
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Signal boxThere is an interlocking tower at Winnipeg on the CPR, called Rugby Tower.
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North American railroad interlocking towers and cabins 
