Relay Nomenclature

Description (Prefix)		Apparatus (Last letter)
A	Approach; Automatic; Relay Contact - Arm	A	Axle Counter
B	Block; Bolt; Relay Contact - Back	B	Block Instrument
C	Checking or Proving; Coding	C	Contact
D	Clear (green) [distant]; Decoding; Relay Contact - De-energised	D	-
E	Light; Lamp; Heat (externally applied); Emergency; Earth	E	Light; Lamp; Earth
F	Fog; Flashing; Feed; Relay Contact - Front	F	Detonator Placers
f	Frequency	f	Fuse
G	Signal	G	Signal apparatus
g	-	g	Lightning Arrestor
H	Caution (yellow) [Home]	H	Capacitor
HH	Preliminary Caution (double yellow)		-
I	AWS	I	Inductor
J	Time (delaying action)	J	Rectifier; Diode
K	Indicating or detecting	K	Indicator electro-mechanical
L	Locking; Left	L	Lock
M	Marker; Magnetic	M	Motor
N	Normal	N	Release; Hand-operated switch; Push-button or key
O	Retarding	O	Resistor; Heater
P	Repeating	P	Lever Latch or Trigger contact
Q	Treadle or Bar	Q	Local coil of a double-element relay; Treadle; Bar
R	Reverse; Right; Danger (red)	R	Relay or contactor
Rx	Receiving	Rx	Receiver
S	Stick	S	-
T	Track Circuit	T	Transformer
Tx	Transmitting	Tx	Transmitter
U	Route [roUte]	U	Unit
V	Trainstop	V	Trainstop apparatus
W	Points [sWitch]	W	Points operating apparatus
X	Audible annunciator; Level crossing; Wrong direction	X	Audible annunciator (bell, buzzer, horn, etc)
Y	Slotting or disengaging	Y	Slotting or disengaging apparatus
Z	Special (to be explained on plan)	Z	Special unit (to be explained on plan)

Source: http://static.scbist.com/scb/uploaded/331_1354714587.pdf

Description (Prefix)
A	Auto button
B
C
D
E	Emergency Replacement Button
F	Ground Frame Release Free
G
H
I	Isolation Overlay Group (not universal)
J
K
L
M
N	Button
O	Override
P	Points
Q
R
RO	(suffix) Yellow exit button
S	(Prefix) Signal (Suffix) White exit button
T	Track Circuit
U
V
W
X	(Suffix) Grey exit button
Y
Z	}