जिंदगी में शादी के अलावा ट्रेन में RAC ही एक ऐसी स्थिति है,जिसमें दो अजनबी साथ निभाने को मजबूर होते है। - Junaid

Please go to a PRS 24 hours prior to the starting time of the train from source. Carry ID card in case of E Ticket. Give an application to change the boarding. You can change the boarding point to any station which falls between your destination. The rule says it has to be done from the station where the ticket is purchased from, but I have done it at other stations and for internet booked tickets it can be done at any PRS. The rule as it is mentioned in indianrail is:
A specific request must be made in writing at the station from where the ticket is purchased and reservation made atleast 24 hours before the scheduled departure of the train...
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from the starting station.
The Railway Administration reserves the right to use such accommodation from the originating station up to the station at which the passenger is due to entrain.
No refund will be permissible for the portion of journey not performed by the passenger.