Awesome site on signals and interlocking towers....

That is interesting seeing the interlocking boards. I also found this online, shows what horn signals you should make for each situation:

The required whistle signals are illustrated by "o" for short sounds and "-" for longer sounds:

Sound Indication

(1) Succession of short Use when an emergency exists, or persons
sounds or livestock are on the track. When crews on
other trains hear this signal, they must stop
until it is safe to proceed.

(2) - When stopped: air brakes are applied,
pressure equalized.

(3) - - Release brakes. Proceed.

(4) o o Acknowledgment of any signal not
otherwise provided for.

(5) o o o When stopped: back up. Acknowledgment
of hand signal to back up.

(6) o o o o Request for signal to be given or repeated or
not understood.

(7) - o o o Flagman protect rear of train.

(8) o o o - Flagman protect front of train.

(9) - - - - Flagman may return from west or south.

(10) - - - - - Flagman may return from east or north.

(11) - - o - Approaching public crossings at grade with
engine in front, start signal not less than 1/4
mile before reaching crossing, if distance
permits. If distance does not permit, start
signal soon enough before the crossing to
provide wan-dng. Prolong or repeat signal
until engine occupies the crossing.

Use this signal to warn employees when
view is restricted.

(12) 0 - Inspect brake system for leaks or sticking
Thanks for posting! This is good stuff too which can make our operating sessions more realistic.
JCitron, loving that first link, especially the section on interlocking machines.

Joseph Hoevet has made several charts of signal rules from several different railroads that are really good reference and very nicely laid out. THey're meant for Train-Sim, but the actually railroad charts are pretty accurate.