HOW IT ALL WORKS - All Aboard: The Complete North American Train Travel Guide - page 175

stations they pass, from work gangs laying new track, and from
other official sources using the railroad frequency.
When members of the onboard service crew need to com-
municate, they most often use a closed-circuit intercom or the
train's public-address system. Sometimes they use both, as when
you hear something like the following over the train's PA system:
“Conductor to the IC, please.” That could be just about any-
thing—perhaps a passenger in one of the coaches has asked to be
upgraded into a sleeping car and the car attendant there needs to
ask the conductor if there are any unoccupied roomettes.
Trackside Signs
During your railroad journey, you'll see a lot of signs beside the
track, obviously meant to communicate information to the crew
in the head end. Once explained, the meanings are pretty obvi-
ous. I found that knowing what those signs mean added to the
interest and enjoyment of my train trips.
While all railroads use signs at trackside to convey the same
information to the engineer, there are some differences in the
appearance of those signs from one railroad to another—just
another reason why engineers have to qualify on each route.
Here's a rundown on the most common railroad signs.
Three typical examples of railroad mileposts.