Probably use a letter for each zone like real-life
Example:SC/SV/SHB use S, meanwhile UFC keeps U
LSR/Edgemead use R
Newry Zone use N
Airport Zone use A, no matter the trains are going to SAC , T1-T2 Loop or T3, as well as James St.(Since it is quite close to the airport)
Berrily and Greenslade use same letter, etc.