Before radio communication came into wide use in the 1960's, a locomotive's whistle was an important tool in conveying information to other employees, both on and off the train, and many signals were on the books.
The General Code Of Operating Rules, used by many railroads, contains the following list of whistle signals and their meanings:
Note: "o" denotes a short sound; while a "-" is for a longer sound.
- When stopped, air brakes applied, pressure equalized.
- - Release brakes, proceed.
0 0 Acknowledgement of any signal not otherwise provided for.
0 0 0 When stopped, back up; acknowledgement of hand signal to back up.
0 0 0 0 Request for signal to be given or repeated if not understood.
- 0 0 0 Flagman protect rear of train.
0 0 0 - Flagman protect front of train.
- - - - Flagman may return from west or south.
- - - - - Flagman may return from east or north.
- - 0 - Approaching public grade crossing.
0 - Inspect brake system for leaks or sticking brakes.
A series of short blasts is sounded in an emergency.
Today, the only signals you're likely to hear regularly are the grade-crossing warning (which is also often used to warn employees or others on the tracks); two (or three) shorts to indicate the engineer has received a signal to start the train forward (or backward); and one long blast when a train is approaching a station on a track next to a platform.