Towers and Interlocking Plants

Towers and Interlocking Plants. . , Where railroads cross at the same level ("at grade") or another route branches off, there must be some means to prevent collisions., At one time, nearly every junction had an adjacent tower housing an operator. The tower operators controlled the track switches and signals governing the route of each train through the junction. The controls were "interlocked" to prevent potential conflicts; if one train was given a "Clear" signal, the controls physically prevented a "Clear" indication on any opposing route. Early towers had a series of large mechanical "Armstrong" (hand-thrown) levers, whose motion when thrown was transmitted by a series of moving rods to change the position of track switches. Modern interlocking plants used electric or pneumatic power rather than hand-thrown levers., By the late 1900's, manned towers had nearly disappeared except at busy urban terminals. Today, a dispatcher who may be hundreds of miles away can control all switches and signals electronically.