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

If you already have a ticket, and you date of journey is later, you can approach the PRS window and askfor extension of ticket: it will be done provided seats are available to the new destination. If you have booked on IRCTC, then you have to rebook; the other alternative is to board the train and request the TTE to extend the ticket and pay the difference in the fare between the two stations. Good wishes.

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