HOW IT ALL WORKS - All Aboard: The Complete North American Train Travel Guide - page 181

mile. If the train comes up to a red signal en route and the engineer
is unaware of the reason he is being told to stop, he can call the
dispatcher and say, “I've got a red signal at milepost 172 dot 7.”
The track over which your train is running is divided into sec-
tions, called blocks . There's a signal located at the beginning of
each block to control the rail traffic moving through that block.
Sometimes the distance between signals can be many miles, while
at other times it might be just a matter of a few hundred yards.
How frequently the signals appear depends on how far ahead
the engineer can see, where another track joins the main line, the
location of a switch, and other factors. Generally, signals are set
to keep a following train at least two blocks apart from the train
up ahead.
For many years, signal lights were operated by electricity car-
ried through wires strung on poles running alongside the track.
In many areas, you can still see those poles passing by outside the
train window. More recently, especially since welded rail is used
more commonly, the electric impulses used to operate the signals
are actually carried to them through the rails themselves.
In most cases it's pretty obvious, but here's what those signals
mean:
• Green (also called a clear signal): Proceed at normal speed.
• Yellow : Approach (meaning pass the signal) and be prepared
to stop at the next signal.
• Red : Stop and proceed at slow speed (usually 15 mph).
• Absolute Red : Stop and do not proceed.
There are a number of ways to distinguish an absolute red
from a normal red signal—typically a second red light will be
illuminated or a single red light will be flashing.