The General Code of Operating Rules

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
encountered is Stop or Stop and Proceed.

 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
route is Approach.

 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