MIDWEST INTERLOCKING TOWERS
From 1889 to 1913, the earliest interlockings were registered with the Railroad and Warehouse Commission of Illinois, and assigned numbers in the order of registration. The Railroad and Warehouse Commission was later succeeded by the Illinois Public Utility Commission, and now the Illinois Commerce Commission. List is from various Annual Reports of the Commission.
The earliest interlockings were registered with the Railroad Commission of Indiana, list is from the 1908 Annual Report of the Commission.
The earliest interlockings as reported by the Commissioner of Railroads, State of Michigan, and the successor, the Michigan Railroad Commission. List is from various Annual Reports of the Commission.
Zoomable Google Maps marked with present and past railroad junctions, crossings, and towers.
The number of active interlocking towers is dwindling, as more control is transferred to CTC installations. But Chicago still has one of the highest concentrations of active towers.
Chicago's Interlockings (Background)