The Pennsylvania Railroad was one of the first railroads to replace semaphore signals with position-light signals.
Such signals, which featured a large round target with up to eight amber-colored lights in a circle and one in the center, could be lit in various patterns to convey different meanings, were more visible in fog, and remained effective even when one light in a row was inoperative.

Signal aspects, or meanings, were displayed as rows of three lit lights.
The aspects corresponded with upper-quadrant semaphore signal positions: vertical for "proceed", a 45° angle rising to the right for "approach", horizontal for "stop", a 45° angle rising to the left for "restriction", a "X" shape for "take siding", and a full circle (used in electrified territory) for "lower pantograph".
Additional aspects were conveyed with a second target head below the first, either a single light, a partial target, or a full target.
Separate Manual Block signal aspects existed as well.

In later years, interlocking home signals north and west of Rockville (near Harrisburg) were modified so that the two outside lights in the horizontal "stop" row had red lenses; the center lamp would be extinguished when the signal displayed "stop".
Such "red-eye" lenses were also temporarily installed at Overbrook Interlocking near Philadelphia.

Starting in the late 1920s, the Pennsy installed Pulse code cab signaling along certain tracks used by high-speed passenger trains.
Information traveled through the rails using track circuits, was picked up by a sensor on the locomotive, and displayed in the engineer's cab.
The Pennsy ultimately installed cab signals on its New York-Washington, Philadelphia-Pittsburgh, and Pittsburgh-Indianapolis lines (the latter which was later downgraded by PC and ultimately abandoned by Conrail).
The Pennsylvania also experimented with cab signals without wayside signals, an approach later expanded by Conrail (Conemaugh line) and Norfolk Southern Railway (Cleveland line).
Cab signals were subsequently adopted by several other U.S. railroads, especially on passenger lines.
This technology, advanced for its time, is still used by Amtrak.
The Pennsylvania Railroad was one of the first railroads to replace semaphore signals with position-light signals. Such signals, which featured a large round target with up to eight amber-colored lights in a circle and one in the center, could be lit in various patterns to convey different meanings, were more visible in fog, and remained effective even when one light in a row was inoperative.
Signal aspects, or meanings, were displayed as rows of three lit lights. The aspects corresponded with upper-quadrant semaphore signal positions: vertical for "proceed", a 45° angle rising to the right for "approach", horizontal for "stop", a 45° angle rising to the left for "restriction", a "X" shape for "take siding", and a full circle (used in electrified territory) for "lower pantograph". Additional aspects were conveyed with a second target head below the first, either a single light, a partial target, or a full target. Separate Manual Block signal aspects existed as well.
In later years, interlocking home signals north and west of Rockville (near Harrisburg) were modified so that the two outside lights in the horizontal "stop" row had red lenses; the center lamp would be extinguished when the signal displayed "stop". Such "red-eye" lenses were also temporarily installed at Overbrook Interlocking near Philadelphia.
Starting in the late 1920s, the Pennsy installed Pulse code cab signaling along certain tracks used by high-speed passenger trains. Information traveled through the rails using track circuits, was picked up by a sensor on the locomotive, and displayed in the engineer's cab. The Pennsy ultimately installed cab signals on its New York-Washington, Philadelphia-Pittsburgh, and Pittsburgh-Indianapolis lines (the latter which was later downgraded by PC and ultimately abandoned by Conrail). The Pennsylvania also experimented with cab signals without wayside signals, an approach later expanded by Conrail (Conemaugh line) and Norfolk Southern Railway (Cleveland line). Cab signals were subsequently adopted by several other U.S. railroads, especially on passenger lines. This technology, advanced for its time, is still used by Amtrak.
Pennsylvania Railroad