Poorva Express: हुगली सी बहती तेरी चाल है पूर्वा तू बेमिसाल है - Keshav Singh

You can cancel only 24 hours before the journey time. Not 15 hours. No refund possible in this case. Here are the rules:
REFUND ON TATKAL TICKET :
1) A flat refund of 25% of total fare charged on the ticket, excluding Tatkal charges may be granted on cancellation of confirmed Tatkal tickets which are presented for cancellation upto 24hrs before the schedule departure of train. Thereafter, no refund will be granted on cancellation of confirmed Tatkal ticket except under the conditions prescribed below
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Full refund of fare and Tatkal Charges will be granted on the tickets booked under this scheme in the following circumstances:
1.If the train is delayed by more than 3 hours at the journey originating point of passenger and not the boarding point if the passenger's journey originating point and boarding point are different.
2.If the train is to run on diverted route and the passenger is not willing to travel; boarding station or the destination station or both the stations are not on the diverted route
3.In case of non attachment of coach in which Tatkal accommodation has been earmarked and the passenger has not been provided accommodation in the same class.
4.If the party has been accommodated in lower class and does not want to travel. In case the party travels in lower class, the passenger will be given refund of difference of fare and also the difference of Tatkal charges if any.
5.The booking under tatkal scheme will be done only from the originating to terminating point of the train with boarding facility from intermediate station. However zonal railways have been given the discretions of imposing restrictions on tatkal booking regarding defining trains as end to end or keeping it free or imposing distance restriction etc. Keeping in view the over all utilization of the accmmodations avliable in the train
3) No refund will, however, be permissible on the reservations made under Tatkal scheme in case the coach, in which accommodation under Tatkal scheme has been earmarked, is not attached and the accommodation has been provided to the Tatkal passengers in the normal train services in the same class.
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indianrail.gov.in under Refund heading, look for tatkal...
Good wishes.