Philadelphia Chapter, PRRT&HS
PO Box 5065
Delanco, NJ  08075

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Last Update:  05/29/17

ARTICLE:  Position Light Signals

PRR Position Light Signal Aspects (* = Adopted in 1943)








Proceed; Manual Block Clear

Clear - Block







Proceed approaching next signal at medium speed. Note: Trains may proceed approaching next signal at not exceeding 45 miles per hour at signals displaying a yellow triangle outlined in black. *

Approach - Medium



Proceed, intermediate speed within interlocking limits. Note: Trains may proceed at not exceeding 45 miles per hour within interlocking limits, at signals displaying a yellow triangle outlined in black. *

Medium - Clear


Proceed at medium speed prepared to stop at next signal. Train exceeding medium speed must at once reduce to that speed.

Medium - Approach





Proceed approaching next signal at slow speed. Train exceeding medium speed must at once reduce to that speed.

Approach - Slow



Proceed prepared to stop at next signal. Train exceeding medium speed must at once reduce to that speed.




Train exceeding medium speed must at once reduce to that speed. Where a facing switch is connected with the signal, approach that switch prepared to stop. Approach next signal prepared to stop.




Proceed; slow speed within interlocking limits.

Slow - Clear




Proceed prepared to stop at next signal; slow speed within interlocking limits.

Slow - Approach




Block occupied; for passenger trains, stop; for trains other than passenger trains, proceed prepared to stop short of train or obstruction, but not exceeding 15 miles per hour

Permissive - Block





Proceed at restricted speed.




Stop; then proceed at restricted speed. Note: Freight trains of 90 or more cars or having tonnage of 80 percent or more of the prescribed engine rating may proceed at restricted speed without stopping at signals displaying a yellow disc on which is shown the letter "G" in black. The engineman must be notified as to tonnage and number of cars in train before leaving terminals and when consist is changed en route.

Stop - And - Proceed







Stop - Signal

PRR Position Light Signal System

A Brief History and Explanation of the Signal System of the PRR, Conrail , etc.
(Based on the original article by Edward Waytel)

Early Goings

Electrification of the PRR began in the summer of 1913 with a test of overhead catenary on a section of track east and west of Wayne, Pennsylvania. The test tried two different types of construction. One had round steel poles connected with H beam steel at the top. The other method used messenger wire connected to the poles and guy rods. After trial and evaluation, the guyed poles and wire cross-construction was adopted. Where space was limited, self-standing posts of lattice construction were used. Spacing the signals depended on speed gradient, curvature and spacing of catenary supports (roughly 300 feet). As a result, a general distance of 2,500 to 4,000 feet between signals was used.

Why Position Light Signals?

Semaphores, already used by the PRR, had much to be desired. They were prone to mechanical problems and required high maintenance. In addition, they were unreliable in bad weather.

Compared with color light signals, the position light won for a few reasons. First, the amber color offered little, if any, glare. Second, they were much easier to see in fog and could not have their color distorted by it. Third, distinctive rows of lights made them easy to distinguish in any environment. Fourth, multi-light signals were much easier to view in electrified territory. Fifth, even a burned out bulb in a signal row would not stop an engineer from reading the signal properly.

Progress in long-range daytime viewing was made through the years and experiments with different configurations continued until two rows of four-light signals were used with tombstone-shaped backgrounds starting in 1915.

Sunlight reflection problems for standard signals were resolved by replacing cover glasses with conic lenses having a frosted tip. Reflections from within the signal lights was resolved by redesigning the lamps, relocating the reflectors and painting the steps in the lower quadrants of the lenses. For dwarf signals, problems were resolved by frosting the cover glasses and inserting a fine mesh material between the cover glass and lens of each light.

Later Changes

In 1916, Light-out relays and simplified aspects came into being. Originally, two arms of four lights were employed for all aspects. But, in fact, there were some aspects that only required one row of lights. If a row were to go out, it could lead to a false indication. As a result, a low resistance relay was installed for the upper arm of lights. If the upper arm went out, the lower would extinguish with it. Simplified aspects for those signals installed during the first year of the program helped save electricity and bulbs. By the 1920's, the relays were removed because they were seen as overcautious. (The probability of an entire row of lights going out at the same time is insignificant.)

In 1917, the tombstone backgrounds were changed slightly because of wind resistance problems with signal masts trackside. Other innovations included holes in the backgrounds through which the lights were placed. This made for easier wind and snow passage, simplified maintenance for the mechanism and easy access to the bulbs for changing.

Voltage changes were used for different times of the day from the very beginning of the program. Lights that are too bright during the evening were hard on the eyes and made for difficult reading from a distance. To accommodate summer twilight hours, a third voltage was used to ease transition to evening voltage. The voltages for daylight, twilight and evening were 11, 6 and 4, respectively. (The voltage changes were made by signalmen.) By 1918, simplified aspects (with fewer bulbs) eliminated the voltage issue. This simplification included using three lights for the lower arm in slow speed signals.

The three light signal known by us today started taking shape in 1921. Upper and lower arms were made into the three light configuration because there were no adverse effects on interpretation or distance viewing. The backgrounds changed in shape and size as well.

After 1921, advances in technology allowing for the use of batteries, improved generators and cheaper power helped spread the use of position light signals. Pedestal signals were introduced around 1930 and were used in areas of close clearance; curved boxes allowed for even more clearance. By the late 1940's, 97 percent of the PRR's main and secondary lines were using position light signals.