I am not sure but I think the first 2 numbers represent the category & zone of train
For eg; For mail exp Train Numbers start with 1
For SF exp Train Numbers start with 2, also
Usually CR trains have 2nd digit as 1 and SWR trains have 2nd digit as 6 & SCR has 2nd digit as 7 and the rest...
more... I think is the series of Trains alloted as per their introduction! (Correct if any)