Color Position Light

The color position light (CPL) signal was developed by Frank Patenal, superintendent of signaling of the Baltimore and Ohio railroad, circa 1918. He also developed a proprietary signal aspect system to replace the earlier A.H. Rudd, ARA standard signaling system (PRR-based) then in use. The CPL system was unique in that it was a conceptually original design instead of being an update of an existing system. The CPL system incorporates several design principles that are otherwise unique to North American signaling. Use of the color red only in the case of an absolute stop or restricted speed situation is the most significant characteristic. The other 11 standard possible combinations do not display a red aspect.