"Yellow over Red" would seem to indicate "Approach" -  Proceed not to exceed "Medium" speed (usually ~30 mph ?), prepared to STOP at the next signal (or bulletined STOP sign location) - unless permission is given by the Person-in-Charge to pass that STOP sign at some designated speed - perhaps at "Restricted" speed - Proceed,  prepared to STOP within 1/2 the range of vision [important in bad weather], short of train, obstruction, etc, not exceeding [corporate-specified speed - usually 15-20 mph].  In most cases, stopping a train under those restrictions from 30 mph could be problematic.  It takes time/distance to recognize an "obstruction" and react, in addition to the "braking distance".  Just my observation - am open to education/correction, if I'm mistaken.

Chooch 42: I observed GTW trains  passing this signal at the 30 mph range, thus how I interpreted the rule. If any out there know different, let me know. Yours, E.H.

On most roads this yellow over red track flag indicates that two miles ahead you must be prepared to stop at red flag which is protecting men and/or equipment working within specified limits.  Foreman in charge of above specified limits may give permission to pass red flag at specific location without stopping and proceed through his (her) limits and at what speed and/or any other instructions necessary. 

If red flag not displayed train my proceed (most roads) prepared to stop short of men, equipment, red flag,   Speed varies by road but usually 10mph until entirely by green track flag and/or have been advised where restriction ends.

There is often more to fully comply with this rule but this is the basics.

There are two Standard Rulebooks used in the United States, GCOR or NORAC both came into existence in 1980's and both follow the regulations set forth in CFR 49. 

CSX uses NORAC which does not use the Yellow over Red Board.

GTW must have been using GCOR or similar rules at the time of this picture.

In GCOR There are two types of flag restrictions, one is for Track or Building Structures and the other associated with Form B's

5.4 Flags for Temporary Track Conditions
5.4.1 Temporary Restrictions
Track bulletins, track warrants, or general orders may restrict or stop train movements because
of track conditions, structures or men or equipment. Yellow flags are used to indicate temporary
speed restrictions. Yellow-red flags are used to indicate when a train may be required to stop.
When flags are not displayed, that information will be included in the track bulletin, track warrant,
or general order.
When a restriction spans adjoining subdivisions, separate temporary restrictions may be issued
on each subdivision. Only one set of flags may be displayed in advance of the entire restriction in
each direction.

YELLOW FLAG

 5.4.2 Display of Yellow Flag
A. Restriction Is In Effect
Two Miles Ahead of Restricted Area. Yellow flags warn trains to restrict movement because
of track conditions or structures. To make sure train movement is restricted at the right
location, employees must display a yellow flag 2 miles before the restricted area.

GREEN FLAG

5.4.5 Display of Green Flag
A green flag indicates the end of a temporary speed restriction. If a series of locations requires
reduced speeds, the green flags could overlap yellow flags. When this is the case, employees
must:
• Place a yellow flag before each speed restriction.
• Place a green flag at the end of the last speed restriction.

YELLOW-RED FLAG

5.4.3 Display of Yellow-Red Flag
Two Miles Ahead of Restricted Area. Yellow-red flags warn a train to be prepared to stop
because of men or equipment. To make sure the train is prepared to stop at the right location,
employees must display a yellow-red flag 2 miles before the restricted area.

B. Restriction Is Not In Effect
When a yellow-red flag is displayed and no restriction is in effect as specified by a track
bulletin, track warrant, or general order, a crew member must attempt to contact the
employee in charge of the yellow-red flag. Crew members must be prepared to stop short of
a red flag 2 miles beyond the yellow-red flag. If a red flag is displayed, proceed as outlined
in Rule 5.4.7 (Display of Red Flag). If no red flag is displayed and no instructions have been
provided by the employee in charge of the yellow-red flag:
1. Move at restricted speed.
2. Increase speed only after:
a. A crew member has received instructions from the employee in charge.
or
b. The leading wheels of movement are 4 miles beyond the yellow-red flag, and the
train dispatcher has verified that no track bulletin or track warrant protecting men or
equipment is in effect at that location.

RED FLAG

5.4.7 Display of Red Flag
A red flag is displayed where trains must stop. When approaching a red flag the train must stop
short of the red flag and not proceed unless the employee in charge gives instructions, including
the milepost location of the red flag. A crew member must attempt to contact the employee in
charge to avoid delay, giving the location of the red flag and the track being used. If instructions to
proceed are received before the train stops, the train may pass the red flag without stopping.
If track bulletin Form B is not in effect, instructions must include speed and distance. This speed
must not be exceeded until the rear of the train has passed the specified distance from the red
flag, unless otherwise instructed by the employee in charge.
Displayed Between Rails. When a red flag is displayed between the rails of a track, the train
must stop and not proceed until the flag has been removed by an employee of the class that
placed it.