Railway Color-position signals
A system combining aspects of the color and position systems was developed on the Baltimore and Ohio Railroad in the 1920s and was also applied to the Chicago and Alton Railroad when the latter was under B&O control. The CPLs were first installed as a pilot on the Staten Island Railroad in New York City, a former B&O subsidiary later turned rapid transit line operated by the Metropolitan Transportation Authority. The B&O system used a central round head with pairs of lights mimicking the traditional semaphore positions using pairs of large coloured lights (green |, yellow /, red –) with a lunar white \ also being present sometimes. The main head was surrounded by up to 6 so-called “orbitals” at the 12, 2, 4, 6, 8 and 10 o’clock positions. The function of the main head was block occupancy information with green representing 2 or more clear blocks, yellow 1 clear block and red/lunar white representing no clear blocks. The orbitals would then serve to provide speed information, 12 o’clock being full (authorized) speed, 6 being Medium speed (Limited speed if flashing), 10 being Full to Medium (Limited if flashing), 2 being Full to Slow, 8 being Medium to Medium, 4 being Medium to Slow and no lit orbitals being Slow to Slow.
The B&O CPL system was, and continues to be, the most theoretically sound signaling system in North America. It is the only system of signal aspects used in North America which only displays the colour red for situations involving an obstructed block or interlocking. Also, it is the only system to use the same aspects on high signals as it does on dwarf signals. Despite its advantages in clarity and viability, due to higher maintenance and construction costs it was not adopted by other railroads, and in the 1990s and 2000s CSX was gradually replacing these signals with color light signals, though as of 2006, clusters of them remained, especially on secondary main lines. When the Staten Island Railroad was re-signaled in 2005 the MTA decided to keep and upgrade the CPL system.
The Norfolk and Western as well as Amtrak both used a system which altered former all-amber position lights to ones with colored lenses for visibility purposes. These should not be referred to or mistaken with B&O Color Position Lights. On Amtrak they are officially called Position Color Light although colorized position light would also be accurate.