By decree of royal order, as of the year 1006 C.R. all locomotives must have two whistles with no less than 10 octaves difference in tone, to aid in warning signals, the permittable styles are of the following varieties:

High tones:

High "Bell" Whistle

High chime (3 chamber) whistle of the notes F#, C# and A natural
link to Audio

Low whistles

Low bell whistle

"Hooter" Whistle

Low Chime (6 chamber)

Whistles are to be allocated as follows:

Standard passenger locomotives (Up to 4-6-0): High chime and hooter

Express passenger locomotives (4-6-2 or larger): High chime & low chime

Tank engines, mixed traffic and freight locomotives up to 2-6-2: High bell and low bell

Heavy freight locomotives (0-8-0 and larger): High bell & hooter

The following whistle codes are also to be implemented.
"H" means High tone, L means Low tone, = means long blast, 0 means a short blast, for example H= means a long High blast, and L0 means a low, short blast. What qualifies as long or short is down to driver's discretion, but generally a short whistle is no longer than 1 second whilst a long whistle is no shorter than 1 second:

Departure signals

H0: (When stationary) preparing to move light engine/yard movement

H= H0: Departing with a train

H0 H0: Locomotive moving onto mainline from siding

H0 L0: Locomotive changing direction whilst "light engine"

H= L0 L0 L0 H=: Banking, double heading, or "Top 'n' tail" departure signal (in this case the second engine will repeat the signal to acknowledge)

whilst moving:

H0: (Moving) Preparing to stop (all movements)

L=: Passing through a station via a through line (one that does not have a platform)

L= H0 H0: passing a platform at less than 60mph

L= H= H0 H0 L0 H0: Passing a platform at more than 60 mph

L= L= H0 L=: approaching unguarded crossing

L0 H0 (1 second bursts): approaching a crossing guarded with barriers

Warning

L= H0 L0 H0 L0 H0: passing individuals on or near track that aren't obstructing path of locomotive (repeat until passed, with the L= changed to L0)

L= L0: danger relating to the surroundings (individual or obstruction on the tracks)

H0 H0 H0 H0 H0 (of a "frantic" frequency): danger relating to the train itself (such as a runaway)

H0 H0 H0 L= H0 H0 H0: Fire on lineside (signaled to signalpony)

Request

L0 H0 H0 H0 L0: banker required

H0 H0 L=: train requiring fresh engine at next stopping place

L=...H0 H0 H0: Engine Requiring water

H0 H0 H0...L=: Engine requiring coal

L0 L0 L0 L0 L0: request to pass signal at danger

Shunting (if radios are unavailable)

H0 H0 H0 H0: To indicate that train or light engine has been shunted clear of points leading from one line to another

H0 H0...L0 L0: Shunt from sidings to main line

L0 L0 L0...H=: Switch point in front

L0 L0 L0...L=: Switch point behind

L0 L0 L0 L0: To or from bay or platform lines