Railway Position light signals
A position light signal is one where the position of the lights, rather than their colour, determines the meaning. The aspect consists solely of a pattern of illuminated lights, which are all of the same colour (typically amber or white). In many countries, small position light signals are used as shunting signals, while the main signals are of color light form. Also, many tramway systems (such as the Metro of Wolverhampton) use position light signals.
On the Pennsylvania Railroad, lights were displayed in rows of three, corresponding to the positions of a semaphore blade. Multiple signal heads were used at interlockings where four aspects did not suffice. The Pennsylvania Railroad chose to use position lights to both replace the semaphores and their moving parts as well as because the intense amber light provided superior visibility in adverse weather conditions such as rain or fog. The prototype position lights used rows of 4 lamps in an asymmetric fashion in the style of semaphore blades, but this was later changed to the symmetric 3-lamp system. The first installation of 3-lamp semaphores occurred on the Main Line between Philadelphia and Paoli, in concurrence with the 1915 electrification. These first signals differed from the later ones in that the lamps were mounted separately in front of a tombstone shaped black painted metal backing. Later the lamps and backing were integrated into a single unit.
The Norfolk and Western also adopted PRR type amber position lights, as the PRR had a 33% share in the N&W at the time. Furthermore, the Long Island Rail Road adopted position lights after it was bought outright by the PRR. After the Penn Central merger, the former all-amber position lights were modified with twin red lenses in the upper horizontal position to enhance recognition of Stop signals at interlockings. The N&W also modified its all-amber position lights to include color in the 1950s, and Amtrak fully coloured its inherited position lights (replacing all the amber) starting in the 1980s.