Any train going away from it's base zone/station is called as UP; and any train coming back to it's base/home zone is called as down.
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UP trains usually have odd last digit in train number; while the DOWN trains have even.
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E.g.,...
more... 12009/12010 is homed at BCT (however station in same zone) hence, going away to ADI it is called 12009, while returning it is 12010.