509 Electrical Light Display Block Signal

As railroads developed in the mid-nineteenth century, controlling train traffic first relied on telegraph messages that informed station masters of unexpected schedule changes, which they then passed to train crews. As traffic increased, this method became too slow and cumbersome, leading to the block tower system. On the Northern Central Railroad, almost all towers were hexagonal and two stories high, each controlling a block of track three to five miles long. Mechanical controls in the towers operated local switches and signals by cables running through protective chases or troughs, determining which trains had priority on the main tracks, which had to use side tracks, and the speeds allowed within that block. In the 1930's, electrical control systems, including the 509 electrical light display block signal, began replacing manually operated signal systems, allowing most block towers to close. As the new system was perfected, it greatly improved railroad efficiency and safety, and by using the electrical conductivity of the rails to determine train locations, it advanced to the point that by the mid 1950's a single operator was responsible for fifty or more miles of track. Today, trains are controlled and tracked by global positioning systems.

Maryland · United States