Rockfort Express: திருச்சி அவனது கோட்டை, இரவில் சென்னை வரை வேட்டை, அவன் தான் மலைக்கோட்டை. - Vijay Baradwaj

According to number, all odd train numbers are up and even are down. (Some passengers exempted as they do not have return versions) Often, it has been noted that the train's journey from it's zone/area to the destination is considered up and the journey back to its home is considered down:
E.g. 12603/04 MAS-HYB Express belongs to MAS.
Therefore, 12603 MAS-HYB is considered up and 12604 HYB-MAS is considered down.
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Many exceptions have been pointed out for this again:
12219/20 SC-LTT Duronto belongs to SC but 12219 Up is LTT-SC.
12759/60 HYB-MAS Charminar Express belongs to SC but 12759 Up is MAS-HYB.
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Theories have then come up stating all trains moving:
South - North: Up
North - South: Down
West - East: Up
East - West: Down
.
Exceptions for this rule may also exist, unknown to me. :)

श्रीनिवास जी आपका कहना सत्य है लेकिन एक चीज़ और जिसे आप नहीं देख पाए हैं. गाडी का नंबर सम या विषम (even or odd) होने से अप और डाउन का फर्क जरूरी नहीं है. जैसे 11061/11065 इलाहबाद तक अप ट्रेन है जबकि इलाहाबाद के बाद यह गाडी डाउन हो जाती है.
19038/19040 गोरखपुर / मुजफ्फरपुर से डाउन ट्रेन बन कर खुलती हैं.
इसी तरह आप कुछ गलती कर गए हैं .
पूर्व...
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(east)से पश्चिम (west) की तरफ जाने वाली गाडी अप होती हैं और पश्चिम से पूर्व जाने वाली गाडी डाउन. हावड़ा पूर्व में पड़ता है और यहाँ आने वाली हर गाडी डाउन होती है और खुलने वाली हर गाडी अप.

Ah! Now this reminds of another blog post where a similar discussion was going on. Up and down can be determined on three bases:
1. Even or odd no.
2. Direction of train
3. Zone of the train.
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long (rather prolonged discussion) was going on on this and a few users were fed up with this. One guy posted a pic in which two trains were moving in the opposite directions in two adjacent tracks and wrote "don't know what you guys are discussing: I only know this up and down of trains."