এক রাজ্য, দুই রাণী।নাম তাদের, শালিমার ও আলিপুরদুয়ার রাজ্যরানী - Dip

Q. 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.
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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.
History
In the UK, the convention was that all trains going to London were "up", and all those going away from it were "down". In India, the GIPR adopted this convention with respect to Bombay, and the Madras Railway adopted it for Madras. Interestingly, the East Indian Railway decided to name trains towards its base, Calcutta, "down" trains, perhaps expecting the GIPR to eventually reach Calcutta; this convention was also adopted by the Bengal-Nagpur Railway. On the North Western Railway "up" was to the west, so that trains from Delhi to Amritsar are "up". However, to avoid confusion among trains numbered the same and belonging to different companies or zones, some changes were made.