actually this 'tatkal' feature is useful for long distance trains.
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ex: train 16318 is from JAT to CAPE
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the train starts on Monday' 23.45hrs....that means JAT people can book tatkal on sunday...and where...
more... as WARANGAL(it comes on wednesday) people dont know when tatkal opens for this train and if they chek on tuesday it will be like regret/waitlist....for this sake this feature is added to ARP scheme
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WARANGAL is taken as example for understanding.