There are several categories of signaling protocols. The most familiar ones are Automatic Block Signaling (ABS), Absolute-Permissive Block (APB) signaling, and Centralized Traffic Control (CTC). Whichever signaling protocol is used there is a basic set of signal indications and meanings. Most North American railroads have standardized on the General Code of Operating Rules. Please note that there are many other rules that are specific to a particular railroad. Consult your favorite railroad's employee handbook or timetable for information. Some ABS sequences are illustrated after the rules below. |
Rule |
Meaning |
Indication |
9.50 (230) |
Clear. Proceed with at least two clear blocks ahead. |
Green |
9.54 (234) |
Approach Medium. Proceed prepared to pass next signal not exceeding 40 MPH. This indication basically means that the next signal indication to be encountered is Approach. |
Flashing Yellow |
9.56 (236) |
Approach. Proceed prepared to
stop before any part of train or
engine passes the next signal.
Reduce speed to 40 MPH or less. This
indication means that the next
signal indication to be |
Yellow |
9.57 (237) |
Diverging Clear. Proceed on diverging route not exceeding prescribed speed through turnout. |
Red over Green |
9.58 (238) |
Diverging Approach Medium.
Proceed on diverging route not
exceeding prescribed speed through
turnout prepared to pass next signal
not exceeding 40 MPH. This
indication means that the next
signal indication to be encountered
on the diverging |
Red over Flashing Yellow |
9.59 (239) |
Diverging Approach. Proceed on diverging route not exceeding prescribed speed through turnout prepared to stop at next signal, reduce speed to 40 MPH or less. This indication means that the next signal indication to be encountered on the diverging route is Stop or Stop and Proceed. |
Red over Yellow |
9.61 (241) |
Stop and Proceed. Stop, then proceed at restricted speed. This is known as a permissive stop. |
Red (w/mileage marker on mast) |
9.62 (242) |
Absolute Stop. Stop before any part of train or engine passes the signal. This is known as an absolute stop. |
Red (w/"A" on mast or w/o mileage marker) |
Rules Illustrated (ABS shown)
In scenario #1 above no trains are present.
The rules displayed by the signals are:
Signal Head 1R: Clear Signal Head 1L: Clear Signal Head 2R: Approach Signal Head 2mL: Stop (note that there is no mileage marker on the mast -> Absolute Stop) Signal Head 2sL: Clear Signal Heads 3mR/3sR: Diverging Clear Signal Head 3mL: Approach Signal Head 3sL: Clear Signal Head 4mR: Clear Signal Head 4sR: Clear Signal Head 4mL: Clear Signal Head 4sL: Clear |
In scenario #2 above two trains are present and the Approach Medium feature
is "enabled". The rules displayed by the signals are:
Signal Head 1R: Stop & Proceed (the black bar on the mast indicates a mileage
marker) Signal Head 1L: Stop & Proceed Signal Head 2R: Approach Signal Head 2mL: Stop Signal Head 2sL: Approach Signal Heads 3mR/3sR: Diverging Approach Signal Head 3mL: Approach Signal Head 3sL: Approach Medium Signal Head 4mR: Clear Signal Head 4sR: Stop & Proceed Signal Head 4mL: Approach Medium Signal Head 4sL: Stop & Proceed |
In the scenario #3 above one train is present and the Approach Medium feature
is "enabled". The rules displayed by the signals are:
Signal Head 2R: Approach Signal Head 2mL: Stop Signal Head 2sL: Clear Signal Heads 3mR/3sR: Diverging Approach Medium Signal Head 3mL: Approach Signal Head 3sL: Clear Signal Head 4mR: Clear Signal Head 4sR: Approach Signal Head 4mL: Approach Medium Signal Head 4sL: Clear Signal Head 5mR: Clear Signal Head 5sR: Stop & Proceed Signal Head 5mL: Clear Signal Head 5sL: Stop & Proceed |