second and third digit combinations of train no. indicates them zone.
24 says NR /NCR train
23 says ER /ECR train
26 says SR /SWR train
27 says SR /SCR train
29says...
more... WR /NWR train
30 says ER train
41 says NCR train
but this system is for railway understanding only so in case of shatabdi, janshatabdi, duronto and specials zones use more nombers