So as HWH-ASR Mail is the Oldest Train running between HWH & ASR, thus it's route is also the oldest between the 2 cities and has developed over the years as follows :-
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Amritsar - Ludhiana -> 1868
Ludhiana - Saharanpur -> 1870
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Howrah - Hooghly - Pundooah - Burdwan - Khana - Andal - [ Sitarampur - Madhupur - Jha Jha - Luckeesarai - Patna 1871] { From /blog/post/254281/115 }
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Saharanpur - Bareilly -> 1873
Bareilly - Lucknow -> 1884
Mughalsarai - Benaras(Varanasi) -> 1887
Benaras - Lucknow -> 1893
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[ Howrah - Liluah - Burdwan 1917 ] - Khana - Sitarampur { From /blog/post/254281/115 }
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Thus, by 1918 this entire route from HWH to ASR via PNBE was ready for serving ER Punjab Mail :-)