Where railroads cross at the same level or another route branches off, some means is needed to prevent collisions. Nearly every junction once had an adjacent tower housing an operator who controlled the track switches and signals governing each train’s route through the junction. These controls were interlocked to prevent conflicts, so that if one train was given a Clear signal, the controls physically prevented a Clear indication on any opposing route. Early towers used large mechanical Armstrong hand-thrown levers, whose motion was transmitted by moving rods to change track switch positions. Modern interlocking plants used electric or pneumatic power instead of hand-thrown levers. By the late 1900's, manned towers had nearly disappeared except at busy urban terminals, and today a dispatcher who may be hundreds of miles away can control all switches and signals electronically.