experimented installing red lenses in the horizontal position of the upper head to help increase the at distance visibility of absolute Stop signals at Overbrook interlocking. Under the Penn Central and later Conrail it became standard practice to add these red lenses to high position lights and even some pedestal signals. The Norfolk and Western modified its signals to use red and green lenses in the upper head Stop and Clear positions and yellow lenses everywhere else. In the 1980s Amtrak modified most of its former-PRR position lights to use the equivalent color light colors in all of the positions of both heads. Internally referred to as position color lights, these are not to be confused with color position lights described below, which while functionally similar are structurally considerably different. New PRR type position lights continued to be installed up until the 1980s on former Conrail systems. Today most of the old PRR position lights are slowly being replaced by modern color lights, but Amtrak, SEPTA and the LIRR continue to install new position lights (Amtrak's being of the colorized variety). US&S no longer manufactures position light equipment, but updated models from Safetran continue to be available. Color position light The color position light (CPL) signal was developed by Frank Patenal, superintendent of signaling of the Baltimore and Ohio (B&O) 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. The CPL consists of a central position target with up to four pairs of doublet lens units around the perimeter of the background disc. The lens units are spaced at 45-degree axes using the positions: green |, yellow /, red—and a lunar white \ for restricting also being present in some installations. The main head is surrounded by up to 6 markers at the 12:00, 2:30, 4:30, 6:00, 8:30 and 10:30 o'clock positions. The function of the main head was block occupancy information with