An “o“ indicates a short sound, and “–“ is a long sound:
o Applying air brakes while standing.
– – Proceeding. Releasing air brakes.
o o – Acknowledging a flagman’s stop signal.
o o Acknowledging any signal not otherwise provided for.
o o o Backing up.
o o o o Calling for signals.
– – o – Approaching public highway grade crossings, multiple crossing, tunnels, yards, standing trains or anywhere else railroad workers might be at work.
o Approaching passenger station.
o – Inspect train for a leak in brake pipe system or for brakes sticking.
– o When running against the current of traffic. Also used when approaching stations, curves, or other points where view may be obscured. Approaching passenger or freight trains and when passing freight trains.
* Source: Wikipedia