Who says South India does not have Gatimaan. It is called Vaigai Express. - Thamizh Maran

Can anyone suggest that is it possible to buy 'Unreserved General Tickets' for the journey starting from the station 'B' to 'C' from the station 'A' itself?
Let me try to explain the requirement… Actually I need to travel to Lucknow from Ghaziabad but have reserved ticket up to Kanpur only. I will need to travel from Kanpur to Lucknow with general ticket; for this I would prefer to buy ticket from the originating station (Ghaziabad in this case) itself instead of from Kanpur. Because at Kanpur I may face a situation where there is a train departing in a few minutes but no ticket; and you know how easy it is to buy a general ticket at large stations like Kanpur...
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Central. :)
If it is possible then is/ are there any specific condition(s) or protocol(s) to be followed for the same? Will the ticket dispatcher require any specific details for this? Any constraint regarding validity (period) for these kind of tickets?