Pennsy and N&W Style PL Signals

RAILROAD SIGNALS of the U.S.

POSITION LIGHT SIGNALS
as found on the Pennsy
the Norfolk & Western, and the Lehigh Valley

Early Position Light Signals
Tombstones

The Pennsylvania Railroad started experimenting with a radically new signal design in 1915 (see my timeline). This signal consisted of rows of yellow lights, in an attempt to recreate the position nature of semaphore signals.

The decision to find a new signal followed the start of electrification in 1913, once they found it was difficult to see semaphores thru the catenary wires (despite this problem, the New Haven stayed with semaphores).

The design of these new signals also coincides with the development of a pale yellow "fog penetrating" lens by Corning.

The original Position Light (PL) signal consisted of rows of four lamps, with one being common to all three aspects, as shown to the left. It appears from this drawing, that the central housing, nicknamed the "spider", is close in design to the ones used on current PL signals.

Below is the only photo I have been able to locate of the original tombstone signals. I don't remember which book it's from, but I found it through Google books (in fact, this and all of the drawings are from it). Below it, are two charts of the signal aspects and their meanings.

The PL Lamp

The following diagram shows the principle behind the design of the PL-5 lamp. This is one of the few lamps that has a reflecting surface behind the lamp to (a) increase the brightness and (b) focus the light beam.

The design of the outer contoured lens is to reflect incoming light (from an engine, for instance) off center so it does not enter the inards. The tip is frosted to increase the effect. This lens is yellow in color, which optimizes light transmission through the fog, which explains why "real" fog lights on cars are the same color (not white like the "fog lights" on my Avalanche).

Behind the yellow lens is another reflecting lens for the incoming light which managed to make it through the outer lens.

Behind that is yet another "lens" to further reduce the effect of stray light entering the lamp off-axis. This lens is called a "phankill" unit, and is nothing more than a flat black painted honeycomb material. Being painted flat black, it will absorb light that strikes its surface. Although it will also absorb light from the bulb, the majority of light from the bulb will be parallel to the viewing axis (and honeycomb material), thereby passing through with little attenuation.

Below my drawing is a cross section of an earlier version of this lamp.

PL Signals

The Pennsy, in 1918, simplified the Tombstone signal to pretty much the design that exists today on Amtrak and the Norfolk Southern, although Amtrak started the transition to colorized versions in 1988, eliminating the center lamp.

A signal in Columbia PA, just south of Rt 30 along the Susquehana River. This signal controls a siding where is runs into the mainline. Pictures taken SEP2003. Note that signal has not been colorized. The photo at the top of the page is also from this set.

A nicely restored Pennsy PL signal at the Railroad Museum of Pennsylvania in Strasburg, until you notice that they do not have the proper lenses in the lamps. The hoods also look to be a little on the short side.

"Miniature" PL's

Small Pennsy PL's come in two varieties.

One is the dwarf PL, as shown to the left. These dwarf signals come in both "right-hand" and "left-hand" versions, where the rounded side can be on either the left or right side. Altho I guess the Pennsy could have placed these signals in a square housing, they placed them in "rounded corner" housings. One speculation for this is so the signal can be located closer to the track than if a square case was used. This one is at the Strasburg RR.

Dwarf signals can be mounted on the ground, on poles placing them about seven feet high, and on signal bridges. Some transit systems, like the Baltimore Light Rail system, use them as route indicators. In one instance I know of, CP Rail uses them in Pigs Eye yard St Paul MN as a shove signal, indicating to the engineer when the rear of a train is at a certain point at the tail (south) end of the yard.

The other "small form" the PL signal can take is known as a "pedestal" signal. This type of signal places two dwarf signals in one housing, one atop the other. Through a discussion on the Yahoo Railway Signaling group, it is a medium speed signal, limiting speed to 40MPH, even though it can display clear block" (which on the main line with a high signal, would allow the train to go at the posted speed). On the north side of the Wilmington (DE) station, the Pennsy used pedestals on the overhead signal bridge, maybe because of clearance issues with a larger full size head.

Pictures

Pictures from a trip Michael Watnosky and I took recently (Apr 2008) to Wilmington DE are here.
And pictures of the PL signals in Perryville MD are here.