Yes, you only get the double speed yodel on double track lines at AHBs/ABCLs/AOCLs; it's not necessary with full barriers or single lines. There is a newer type called Apello which gives a spoken message instead :
http://www.e2s.com/case-studies/network ... n-soundersThe OP asked about the early crossings. I thought the very earliest ones had bells, or was that just the MCBs? I suspect the type of audible alarm may sometimes be specified in the level crossing order.
GK/RT0300 (Issue One, dated August 1998) specifies :
5.1
When the level crossing operating cycle is initiated, the Audible Warning Unit
shall emit a sound comprising two alternating repeating signals.
5.2
The emitted sound shall alternate at an increased rate where a second (or
subsequent) train approaches during the level crossing operating cycle.
5.3
The sound pattern shall be sufficiently distinct from other systems in the area
of the level crossing (e.g. Pelican, Puffin Road Crossings and TOWS) as to not
cause any potential confusion amongst pedestrians and other road users.
5.4
The nominal audible signal output shall be capable of adjustment in volume to
suit local background noise conditions including selection for differing
requirements of day and night conditions. The adjustment shall be achieved
from a source external to the Audible Warning Unit.
5.5
Permitted variations in the specified power supply shall not lead to obvious
variation of audible characteristics. This shall including any loading effects on
the power supply generated during the Level Crossing operating cycle.
6.1
The two alternating signals shall have frequencies of 800Hz ±1% and 1000Hz
±1% and equal duration.
6.2
The tonal sequence required in 5.1 shall repeat every 0.5s.
6.3
The tonal sequence required in 5.2 shall repeat every 0.25s.
6.4
The audible signal output shall be adjustable to give outputs ranging between
80dB(A) and 60dB(A), measured at a distance of 3m radius from the unit,
taking into account the effects of phase cancellation between units.