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.