For operational reasons, certain trains have priority in running. To help operating staff, they are grouped into classes:[2]
Express passenger train; nominated postal or parcels train; breakdown or overhead line equipment train going to clear the line or returning from there (headcode 1Z99); traction unit going to assist a failed train (1Z99); snowplough going to clear the line (1Z99)
Ordinary passenger train; breakdown or overhead line equipment train not going to clear the line (2Z99); Officers special train (2Z01)
Freight train which can run at more than 75 mph; parcels train; autumn-railhead treatment train; empty coaching stock train if specially authorised
Freight train which can run up to 75 mph
Empty coaching stock train
Freight train which can run up to 60 mph
Freight train which can run up to 45 mph
Freight train which can run at, or is timed to run at, 35 mph or less
Class 373 train
Light locomotive or locomotives
Since 2010, Class 9 has included London Overground passenger services on the East London Line, to avoid confusion with the large number of Southern and South Eastern services.[citation needed] In 2006, some long distance Virgin Cross Country services were allocated 9xxx headcodes to help signallers identify a long-distance inter-regional express passenger service from a local Class 1 express service. This was to help the Virgin Cross Country get higher priority. This agreement ended when the franchise changed to Arriva CrossCountry, and have now reverted to their former Class 1 designation.
The classes' definitions have changed several times since being introduced: Class 3 was formerly used for parcels trains and empty passenger trains, and Class 9 for freight trains without fitted brakes.