Train movements through busy rail junctions were often controlled by operators housed in interlocking towers. The name refers to the interlocking nature of switches, locks, and signals. Typically, the opening or closing of the switch on one track would affect another switch, lock, or signal somewhere else at the junction.
The tower above, built in 1905, still stands near Mitchell and Spring streets in downtown Atlanta. It was part of the Atlanta Terminal Station complex, all of which has long been demolished except for this building.