PRR Position Light (PL) Signals: History, How to Read Them & Legacy
Last revised: August 19, 2026
By: Adam Burns
If one signal silhouette says “Pennsylvania Railroad” to generations of railroaders and railfans, it is the Position Light (PL): a circular black target with rows of pale amber lamps that recreate the geometry of an upper-quadrant semaphore—vertical for proceed, diagonal for approach, horizontal for stop—without depending on red-yellow-green color coding for the basic story. For more than a century after experimental hardware appeared in 1915, that language governed huge stretches of Pennsy iron and then the successor networks that inherited it: Penn Central, Conrail, Amtrak’s Northeast Corridor and Keystone routes, Norfolk Southern on former N&W and PRR lines, and the Long Island Rail Road.
A classic Pennsylvania Railroad–style Position Light stands at Cresson, Pennsylvania, on April 21, 1988, still speaking amber semaphore language long after the Pennsy nameplate had vanished into Conrail. Roger Puta photo.
Featured here is a complete history of the PL system: origins under signal chief A.H. Rudd, the “tombstone” prototypes, the classic three-lamp-per-row head, how a two-head PL plant encodes speed and route, dwarfs and pedestals, the mid-century red-eye / position-color variant, affiliate roads, and why so many pure amber forests are finally giving way to modern color-light and LED heads.
Naming note: Railfans sometimes say “PRR color position lights.” Strictly speaking, the Pennsy’s standard PL is a position system in fog-penetrating amber. Color entered the story later via red stop lenses (“red-eyes”) and Amtrak’s later position-color practice. The Baltimore & Ohio’s contemporary cousin—the Color Position Light (CPL)—is a different design; see the dedicated CPL article for that road.
At a Glance
| Topic | Summary |
|---|---|
| Full name | Position Light (PL) signal |
| Home road | Pennsylvania Railroad; also LIRR (subsidiary), Norfolk & Western (PRR-affiliated), and some Lehigh Valley applications |
| Principal figure | A.H. Rudd, PRR Superintendent of Signaling (speed signaling + PL development) |
| Why invented | Replace / supplement semaphores; improve night and foul-weather reading; maintain readability under new catenary (electrification from 1913) |
| Lens color | Pale yellow / amber “fog-penetrating” glass (Corning / Churchill work circa 1915)—not a traffic-light trio |
| Prototype | 1915 “tombstone” signals: four lamps per row; large sheet-iron backgrounds |
| Classic form | By ~1918–1921: three lamps per row around a common center (“spider”), circular target |
| Reading rule | Lamp positions mimic semaphore blades; normally two heads encode block occupancy + route/speed |
| Small forms | Dwarf PL-4; pedestal PL-5 (stacked dwarfs); rare “domino” compressed heads |
| Colorized stop | From mid-1950s: “red-eye” / position-color lights—red horizontal stop lenses, center often dark for Stop |
| Status | Still present in pockets; wholesale replacement ongoing under Amtrak, NS, and others with modern color-light / LED signals |
Semaphore Thinking in an Age of Electric Light
By the early twentieth century the Pennsylvania already lived by semaphore language. Upper-quadrant blades—vertical clear, 45-degree approach, horizontal stop—were burned into enginemen’s habits. Night indications on many roads still leaned on colored spectacle glasses, but early electric color-light signals needed bright lamps and reliable power. Position lights took a different path: keep the geometry of the blade, paint it in a single highly visible hue, and drive low-wattage lamps that could run for long periods even when local AC was scarce and battery plants mattered.
Electrification sharpened the problem. When the PRR began stringing catenary (notably the Philadelphia–Paoli project era beginning 1913), semaphore blades became harder to read through wires and hangers. The New Haven could keep living with semaphores; the Pennsy chose to invent around the problem. Concurrent Corning research under Dr. William Churchill produced a pale yellow fog-penetrating glass that the PL program adopted—the same optical idea that later made “real” automotive fog lamps amber rather than harsh white.
The PL was also bound up with Pennsy speed signaling philosophy associated with Rudd: aspects that tell not only “stop or go” but how fast to take a route and what to expect at the next signal. That is why a mature PL plant is almost never a single lonely vertical row. It is a conversation between heads.
A.H. Rudd, Tombstones, and the Classic Head
Contemporary and secondary accounts credit A.H. Rudd, the PRR’s Superintendent of Signaling, as the intellectual driver of the Position Light program and of the road’s broader speed-signaling framework. Experimental PL hardware appears in 1915. Fans nicknamed the early targets “tombstones” because of their large, rounded sheet-iron backgrounds—literally rows of four lamps translating semaphore positions into light, with one lamp common to multiple rows.
Those first heads worked optically but paid a mechanical price. The huge backgrounds acted like sails in wind, stressing masts and bridges. By 1917–1918 the railroad refined shapes to cut wind loading and simplified the lamp count. By about 1918 the design dropped to three lamps per aspect row sharing a center pivot lamp; by 1921 the circular target and “spider” mounting familiar from every Midwestern and Mid-Atlantic right-of-way photograph had crystallized. Rudd himself discussed background practice in technical literature of the period (including a frequently cited Railway Signal Engineer treatment around July 1921): lower-head backgrounds were applied selectively because some aspects were only approached at slow speed and did not need long-range contrast.
Hardware manufacture centered on the usual signal-industry giants—Union Switch & Signal foremost in Pennsy territory—with standardized lamp assemblies (PL-1 / PL-2 / PL-3 heads in common talk; PL-5 optics with reflector, yellow outer lens, and “phankill” honeycomb to kill phantom indications from locomotive headlights). Approach lighting, where used, kept dark signals dark until a train needed them—lamp life and power both mattered on a system the size of the PRR.
Installation followed need and capital. Early tests and first territories rode the electrified Philadelphia approaches; systemwide replacement of semaphores rolled through the 1920s and 1930s until Position Lights were as much a Pennsy trademark as Tuscan Red passenger cars and the keystone herald. Main lines, interlocking plants, yards, and branch approaches all spoke amber.
How to Read a Classic PL
Forget traffic-light habits for a moment. On a standard high PL, position is the primary code:
- Vertical row — Clear / Proceed (block and route permitting track speed unless a lower head or rule modifies it)
- Diagonal row — Approach: prepare to stop at the next signal; reduce as the timetable and rules require
- Horizontal row — Stop (absolute or stop-and-proceed depending on number plate, interlocking vs. automatic, and the era’s rulebook)
A full high signal commonly mounts two heads. The upper head carries the basic block story; the lower head modifies for diverging routes and speed—Medium Clear, Approach Medium, Slow Approach, Restricting, and related names that evolved across PRR rulebooks into Penn Central, Conrail, and NORAC language. That two-head thrift was deliberate: the Standard Railroad of the World did not sprinkle three-head stacks everywhere the New York Central might. Slow-speed work often used dwarf or subsidiary indications rather than a third full head.
Key combinations engineers lived by (names vary slightly by rulebook year, but the geometry is stable):
- Clear — vertical on the upper head
- Approach — diagonal on the upper head
- Stop — horizontal on the upper head
- Approach Medium — Approach over Clear (diagonal over vertical): prepare for a medium-speed move at the next signal
- Medium Clear — Stop over Clear (horizontal over vertical): take the diverging route at medium speed
- Restricting / Slow Approach — typically lower-head geometries (often without full round backgrounds on early practice), requiring restricted or slow speed prepared to stop short of train, obstruction, or switch
Two evolutionary wrinkles matter to historians. Early PL practice, following semaphore night-color limitations, long lacked a clean Medium Approach; that indication returned when flashing relays were trusted enough for safety-critical use (mid-1950s)—flashing the lower diagonal to upgrade slow to medium. And from the 1930s–40s onward, higher-speed turnouts in electrified multi-track territory drove the Limited Speed triangle marker, letting 45 mph class routes exist without rebuilding every signal plant for a third head.
Cab signals on the PRR often simplified wayside PL stories into fewer cab aspects—another reason period charts show wayside-to-cab translation tables rather than one-for-one copies.
Position Lights along the ex-PRR main at South Fork, Pennsylvania, April 22, 1988. Two-head amber geometry—upper story, lower speed/route modifier—was still the everyday language of the old Pennsy spine under Conrail. Roger Puta photo.
Upper Heads, Lower Heads, and Backgrounds
Field PL plants are full of “incomplete” looking heads that are actually correct. If a location never needed a particular aspect, the unused lamp sockets received blanking plates. Upper heads may show one, two, or three aspect rows populated. Lower heads are even more varied: full round backgrounds, flat-sided partial rounds, a background only behind the vertical row, a lone diagonal stalk with no disk at all, or a naked single lamp for stop-and-proceed on intermediate automatics.
Rudd-era logic explains much of that thrift. Aspects approached at speed (notably Approach Medium and Medium Clear on the lower head) deserved contrast backgrounds for long-range reading. Slow Approach and Restricting, approached already at low speed, often ran without full disks—cutting wind load and mast cost. When Approach Slow and Medium Approach proliferated in later decades, fuller lower backgrounds became more common because those indications might now be sighted at higher approach speeds.
Bridge and cantilever practice usually hung the upper head above the structure and the lower head in the plane of the bridge—though exceptions exist (both heads above the bridge, odd Northumberland-area specials, and other local puzzles that delight signal fans and terrify anyone writing a “one size fits all” caption).
Dwarfs, Pedestals, and Dominoes
Dwarf PLs (often called PL-4s) use smaller (~4-inch) lenses, historically frosted or lunar white to limit range so they would not be mistaken for high signals. Housings came in left- and right-hand rounded forms; mounting legs and rear access covers evolved between early and later production. Pennsy used them for yard moves, reverse moves, and slow exits onto mains; later users pressed the same silhouette into transit route indicators (notably Baltimore Light Rail) and even shove indicators on other roads. Unlike B&O’s fully aspected CPL dwarfs, PRR dwarfs live in the slow-speed world.
Pedestal signals (PL-5) stack two dwarf-scale heads in one housing. They are medium-speed tools—even a “clear” pedestal reading does not grant mainline track speed the way a high Clear does. Pedestals appear at interlocking ends of sidings, tight terminal approaches (Wilmington is a textbook overhead-bridge pedestal site), and similar constrained locations. Some later pedestals received red-eye stop lenses.
Domino signals compress the lamp geometry so top and bottom rows sit in straight lines—born of terrible sight lines under Chicago Union Station overpasses and a few tight New Jersey clearances. Same PL language; different packing.
Red-Eyes and Position-Color Lights
Beginning with tests at Overbrook, Philadelphia, in September–October 1954, the PRR experimented with replacing the horizontal amber stop lenses with red lenses and extinguishing the center pivot for the Stop aspect. After trials, the practice spread—especially west of Harrisburg in the mid-to-late 1950s—and entered the 1956 rulebook. Fans call them red-eyes or red snake-eyes; proper terminology in later writing is often position-color light (PCL) for the hybrid.
Why bother? Fog on lines such as the Buffalo route, and the desire for a stop aspect that reads unmistakably as stop at distance, dominate the oral history. Red-eyes were generally applied where a signal could display absolute Stop (home interlocking signals), not casually on every automatic. Pedestal red-eyes followed in Penn Central years on selected Buffalo Line and Chautauqua Branch sidings. Amtrak’s later Northeast Corridor “colorizing” of PL territory—sometimes eliminating the center lamp entirely—is a related but distinct modernization thread from the 1980s onward.
Do not confuse these Pennsy/Amtrak hybrids with B&O Color Position Lights, which were designed from the start as colored positional pairs plus marker lamps. Cousins, not twins.
PL vs. B&O CPL
| PRR Position Light | B&O Color Position Light | |
|---|---|---|
| Primary code | Lamp position (amber) | Position and color on the main head |
| Typical color | Fog-penetrating amber (later red stop on red-eyes) | Green / yellow / red / lunar white pairs |
| Speed refinement | Second head + limited-speed markers / flashing | Marker (“orbital”) lamps above and below the round head |
| Era of adoption | Experiments 1915; classic form ~1921 | Mid-1920s (Patenall); first main-line Ohio territory |
| Dwarfs | Slow-speed PL-4 language | Can carry the full high-signal aspect family when fully equipped |
| Successor story | Amtrak / NS / others replacing with color-light & LED | CSX replaced most CPLs with “Darth Vader” LED heads |
Full CPL treatment: B&O Color Position Light signals.
Beyond the Pennsy: LIRR, N&W, and Lehigh Valley
As a PRR subsidiary, the Long Island Rail Road absorbed Position Light practice deeply—training rooms once displayed high PL, dwarf, and pedestal teaching aids that signal fans still drool over in period photos. Parts of LIRR signaling culture still echo the older PRR two-head logic (including operating theories about how Approach Medium and Approach work together through diverging moves).
The Norfolk & Western, long under PRR financial influence, likewise blanketed much of its property with Pennsy-style PLs. In the mid-1960s N&W began its own colorization program, often deleting the center lamp—different from Conrail’s red-eye horizontal treatment west of Harrisburg, but the same instinct: make stop read as stop in color.
The Lehigh Valley, another PRR-connected property, used PL hardware in places because of that affiliation. When writing captions, it is safer to say “PRR-style Position Light” than to assume every amber circular head west of New York was lettered Pennsylvania.
Successors, Removals, and What Remains
Penn Central inherited the forest; Conrail maintained and selectively red-eyed it; Amtrak and Norfolk Southern became the long-term caretakers on the highest-profile routes. Altoona’s famous PL stands came down in the early 2010s. Columbia, Lancaster approaches, Mapleton, Huntingdon, Perryville, Halethorpe, and dozens of other classic photo locations have seen pure amber heads give way to Safetran/US&S color lights or LED modules—sometimes leaving a Safetran PL-lookalike head as a transitional oddity.
Yet the system is not extinct. Pockets of classic and red-eye PLs remain on secondary mains, connectors, and terminal tracks; museum and tourist lines display restored heads (the Railroad Museum of Pennsylvania’s example is well known, lens accuracy debates included). For photographers, the rule is the same as for B&O CPLs: shoot them while the circular targets still own the skyline.
Modern replacements are safer to maintain and easier to stock, but they erase a visual language that trained three generations of enginemen to read geometry before color. That loss is why PL pages—and CPL pages—belong in the infrastructure library beside bridges, interlocking towers, and catenary.
Related Reading on American-Rails.com
- B&O Color Position Light (CPL) Signals
- Railroad Signals hub
- Rail Infrastructure
- Pennsylvania Railroad
- Norfolk & Western
Signal rules and aspect names vary by railroad, rulebook year, and special instruction. This article is a historical and technical overview for enthusiasts and researchers—not an operating document. Always defer to the carrier’s current timetable and rules.
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