Also included is information on the earliest CTC installations.
Many interlocking towers served multiple railroads, especially where different railroads crossed. Information on towers serving multiple railroads is not directly duplicated, but instead linked to a relevant page for the primary operating railroad. Information often is not included for minor branch lines, but all known towers are identified on at least one of the lines served. Information on the actual operating railroads is as accurate as possible, but not always available. Often when there is uncertainty and one of the railroads is an obscure long abandoned railroad, the tower is more likely listed with the more familiar railroad.
In some of the larger towns with multiple interlockings, information is not always clear on the identity of the interlockings, due to historical inconsistencies in the naming of interlockings. Also in Michigan, there were numerous crossings involving electric and "steam" railroads. And because the electric railways ceased operations earlier than in Illinois and Indiana, it generally was impractical to include these interlockings. This information is not included for the more complex fringe border areas, of Evansville, Louisville/Jeffersonville/New Albany, and Duluth/Superior. The St. Louis area is included, thanks to the fine "Historical Guide to Interlocking Towers", available from the Terminal Railroad Association of St. Louis Historical and Technical Society.
Miscellaneous information on interlocking towers, including state records on the earliest interlocking towers constructed, and additional information on interlockings in the Chicago area.