Am furnishing below an account of the differentiation between UP and DOWN trains Courtesy:IRFCA. But as the write-up clearly mentions, the exceptions are numerous and therefore it would be madness to find a "method in this madness"
Usually, of the 2 numbers allocated to a pair of trains, the UP train bears the odd number and the Down train the even number. But even there there are exceptions like 12810 UP Howrah Mumbai Mail (via Nagpur) and 12809 DN Mumbai Howrah Mail (via Nagpur).
Up and Down Trains
Q....
more... Trains are often referred to as going "up" or "down" —what does this mean?
Down refers to a train travelling away from its headquarters (i.e., the homing railway) or from its Divisional headquarters, whichever is closer. Up refers to a train travelling towards its headquarters or divisional HQ, whichever is closer. Eg, 2903 DN Frontier mail is down travelling away from its HQ (Bombay) and from the division that homes it (Bombay division) and similarly 2904 UP refers to it travelling back towards Bombay, its HQ and division. Usually the numbers for the "up" and "down" trains differ by just 1.
As for the Shatabdis, the odd number indicates the Down run and the even number for the Up run. E.g. New Delhi - Bhopal Shatabdi (the first Shatabdi) is numbered 2001 DN (away form Delhi) and 2002 UP (towards Delhi).
There are numerous exceptions to this scheme! Beware if you are trying to find method in the madness. Numerous inconsistencies and oddities arise near zonal or divisional boundaries. At Howrah, the Kalka Mail was numbered 1 Up / 2 Down and the Howrah - Bombay Mail as 2 Up / 1 Down to avoid confusion.
Interestingly, on the Ring Railway in New Delhi, the clockwise direction is marked "up" in the timetables.
DONT GO TOO DEEP INTO THE MATTER.