The Pennsylvania Railroad had only 12 aspects in its signal rules in 1950, which satisfied all needs.
Rule Number, Name, and Indication
Rule 280, Clear-Block: Proceed, manual block clear. Displayed at the
entrance of an unoccupied manual block.
Rule 281, Clear: Proceed. Displayed at the entrance of an interlocking or
automatic block, or by a distant signal protecting an isolated facing-point
Rule 282, Approach-Medium: Proceed, approaching next signal at medium
speed. Displayed before a Medium-Clear aspect, or before an Approach aspect in
three-block signaling. If a yellow triangle is mounted on the signal, the
medium speed is 45 mph instead of 30 mph. The triangle is a 45° triangle
with one leg horizontal, and the hypotenuse to the upper right. It is yellow
with a black border.
Rule 283, Medium-Clear: Proceed, medium speed within interlocking limits.
Displayed when train is to use a switch permitting medium speed of 30 mph or 45
mph if a yellow triangle is displayed as in Rule 282.
Rule 285, Approach: Proceed prepared to stop at next signal. Train
exceeding medium speed must at once reduce to that speed. Displayed before any
aspect except Rules 280, 281, and 282, and Rule 283 unless the signal can
Rule 285A, Caution: Train exceeding medium speed must at once reduce to
that speed. Where a facing-point switch is connected with the signal, approach
that switch prepared to stop. Approach next signal prepared to stop. Displayed
to protect a facing switch lined for the diverging route, when not interlocked.
Rule 287, Slow-Clear: Proceed, slow speed within interlocking limits.
Displayed when a low-speed route is clear to the next signal, which must be
displaying Slow-Approach or better, or to interlocking exit.
Rule 288, Slow-Approach: Proceed prepared to stop at next signal. Slow
speed within interlocking limits. Displayed at the entrance to a slow-speed
route at an interlocking.
Rule 289, Permissive-Block: Block occupied. For passenger trains, stop. For
trains, other than passenger trains, proceed prepared to stop short of a train
or obstruction, but not exceeding 15 miles per hour. Displayed at the entrance
to an occupied manual block when a train, other than passenger, may proceed
Rule 290, Restricting: Proceed at restricted speed. Displayed at an
interlocking at the entrance to any route except a high or medium speed route.
Does not guarantee indefinite progress (track may end or be occupied).
Rule 291, Stop-and-Proceed: Stop. Then proceed at restricted speed. Freight
trains of 90 or more cars or having tonnage of 80 per cent 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.
Displayed at the entrance to an occupied automatic block.
Rule 292, Stop-signal: Stop. Displayed at any point beyond which a train
may not proceed, as at the entrance to an interlocking, or at the entrance to
an occupied absolute manual block.