पूजा सुपरफास्ट - हर रोज़ चलूँ मैं तेरे साथ मंज़िल मंज़िल - by Shubham Yadav

The basic concept of UP/DN train emerges from UP/DN direction of a particular railway Route....
Generally MSL( mean see level) is taken as a criteria for deciding Up/Dn direction of railway routes.. (But there are exceptions).
As an example...DLI to BCT, DLI to BVT, DLI to HWH, ASR to MGS, FZR to DLI, KALKA to DLI are all DN lines as Elevation of starting stn on all these routes has Higher MSL...
Similarly...
more...

the reverse direction for these routes are UP lines..
If a train originates / terminates within a specified route , It is called Up/Dn exactly as per Route direction. But many trains entering a particular route from other zones/routes may not match with Up/Dn terminology of that route.
UP trains are mostly given ODD numbering ,while DN trains are mostly given Even numbering ( However , again there are exceptions)