HOW IT ALL WORKS - All Aboard: The Complete North American Train Travel Guide - page 177

for that in chapter 6, “Life Onboard.” On some routes, curves
in the track may have been straightened to permit higher speeds;
when that happens, the old mileposts might not be exactly one
mile apart. If you want to be really accurate, check the speed sev-
eral times over any given stretch of track.

Whistle Posts
You'll notice these right away, because there are a lot of them—
a black “W” on a small white sign right at trackside. They tell
the engineer to blow the whistle
because there is a grade cross-
ing ahead—that is, a place where
automobile traffic crosses over the
tracks. These signs are placed at
varying distances from the grade
crossing so that trains traveling at
the maximum speed limit for that
stretch of track will give adequate
advance warning of its approach.
Slower trains will either wait a few
seconds after they pass the whistle
post before sounding the whistle
or they will whistle several times.
In some parts of the country, these
signs look very different, like a
paddle stuck in the ground. On
the paddle's blade there are three
horizontal stripes and a dot. This
is a visual reminder to the engineer
of the traditional whistle signal at
grade crossings: long, long, short,
long. (See “Whistle Signals” on
page 182.)

This is also a whistle post, but it's
a visual depiction of the traditional
warning whistle used for grade
crossings: long, long, short, long.