Railroads use signals to control traffic and give crews permission to use the track. One notable style was the position light signal, first used on the Pennsylvania Railroad in 1915, which used three lights in changing rows instead of a single light to indicate the signal aspect. Originally all the lights were yellow, but by 1959 they had changed to red, yellow, and green depending on the aspect. In this system, a vertical position meant proceed or clear, an angled position meant approach and prepare to stop at the next signal, and a horizontal position meant stop. Secondary heads were used for upcoming signals or side tracks. Because the style relied on rows of lights, it could still be read if one bulb was out, and it was easier to read at speed or in bad weather. Only three U.S. railroads used this style: the Pennsylvania Railroad, Norfolk and Western, and Baltimore and Ohio. Their successors, Norfolk Southern Corporation and CSX, are replacing position light signals with standard modern vertical color lights. The first color light signals were installed during the 1920s with the Automatic Block System, or ABS, to regulate trains on the main track between Richmond, Va. and Hamlet, N.C. As a train passed a signal, it automatically changed from green to red to show the block was occupied, then changed from red to yellow and back to green after the train left the block. During the 1940s, Centralized Traffic Control, or OTC, was installed over the Seaboard main lines and transformed train operations. Dispatchers could monitor main and side tracks for a section on panels that showed train locations, and by radio they authorized trains to operate between locations and changed switch directions from hundreds of miles away. Seaboard became one of the most advanced railroads through CTC, which eventually controlled over 1,700 miles of track. A signal mast with two different heads would have been placed near a switch for a passing siding or spur track, with the upper three-light head controlling the main line and the lower head controlling access to the siding.