1Z18 is the train's headcode.
The first figure denotes the train type:
0 Light engine
1 Mail, express passenger and certain special trains
2 Stopping passenger
3 90mph freight (in the former Southern Region, "express" empty coaching stock moves that need to get to another station to start another service rather than returning to the depot. )
4 75mph freight
5 Empty mail vans and empty stock (ECS)
6 60mph freight
7 45mph freight
8 35mph freight
9 Eurostar passenger
The second figure denotes the destination Network Rail zone (former British Rail region), when the train is travelling between zones, or that the train is a special outside division.
E Eastern
I outbound passenger trains via the Channel Tunnel
L Anglia
M London Midland
O Southern or trains through the Channel Tunnel to London
S Scottish
X "Out-of-gauge" trains, or the Royal Train
V Western
Z Specials, works trains, sandite trains, etc.
G Some specials NE England & S Wales
A,B,C - London
Others do exist, for other journeys, or trains within area.
The last 2 digits are individual numbers for the train.
Note, these do not fully identify a train, different trains can have the same headcode.