When you book an eticket, and you do not travel on a confirmed ticket, you can ask for refund of the amount by filing TDR online by giving the PNR number. Here are the rules from irtc.co.in
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e-Tickets can be cancelled on Internet at this website till Chart preparation of the train .Cancellations are not
allowed at face to face Railway Counters....
more...

If the user wishes to cancel his e-Ticket, he can do so till the time of
chart preparation for the train, which is normally 4 hours before the scheduled departure of the train from the
train originating station. For train starting up to 12 noon the chart preparation is usually done on the previous night. He
can log on to www.irctc.co.in and go to "Booked Tickets" link and select the ticket to be cancelled and can initiate
the cancellation by selecting the passengers to be cancelled. Cancellation would be confirmed
online and the refund would be credited back to the account used for booking as for normal Internet tickets.
If there is any partial cancellation of ticket, please ensure that a fresh e-reservation slip (Electronic Reservation Slip) is
printed separately as done for the original ticket.
To take you step by step and familiarize you with the procedure
for canceling of e-tickets, please click HERE
Cancellation of e-Tickets before chart preparation of the train:

If a confirmed ticket is cancelled more than 24 hrs before the scheduled departure of the train, flat
cancellation charges shall be deducted @ Rs.70/- for AC First Class/Executive Class, Rs.60/- for AC 2
Tier/AC 3 Tier/First Class/AC Chair car, Rs.40/- for Sleeper Class and Rs.20/- for Second Class.
Cancellation charges are per passenger.

If a confirmed ticket is cancelled within 24 hrs and upto 4 hours before the scheduled departure of the train,
cancellation charges shall be 25% of the fare subject to the minimum flat rate mentioned in the above clause.

Less than 4 hours before the schedule departure of the train upto chart preparation of the train, 50% of the
fare paid subject to the minimum cancellation charges.

Note the Chart preparation time is the time for the chart preparation from the Train Originating Station or
from the Previous chart Preparation station.
Cancellation of e-Tickets after the preparation of Reservation charts:
For Normal User:- E-ticket cannot be cancelled after chart preparation. Users are requested to use the online TDR filing for such cases and track Status of the refund case through
tracking service provided by IRCTC. Such claims can be filed from next date from the journey date to 30 days from the date of journey. For filing online TDR , select
the "File TDR" Link in the left panel under the "My Transactions" Menu. For additional information regarding TDR, Select the "File TDR Procedure" in the left panel under the "General" Menu. IRCTC will forward the
claim to Concerned Railways to process the refund and money of refund amount will be credited back to same account through which payment was made after receiving the same from the concerned Railways. For any other clarifications please mail to etickets@irctc.co.in
For Agents :- E-ticket cannot be cancelled after chart preparation. Whenever Agents received such a request from their customers they are required to send a mail to etickets@irctc.co.in containing following details.
PNR No.
Transaction ID :
Train No.
From:
To:
Date of journey:
S.No.
PASSENGER NAME
AGE
Reasons for Filing TDR
IRCTC will file TDR and forward the claim to Concerned Railways to process the refund and money of refund amount will be credited back to Agents account through which payment was made after receiving the same from the concerned Railways. For any other clarifications please mail to etickets@irctc.co.in Please refer to File TDR Procedure
For Tatkal Tickets Booked as e-Tickets:
A flat refund of 25% of total fare charged on ticket, excluding Tatkal Charges is granted on cancellation of
confirmed Tatkal tickets, which are cancelled up to 24 hrs. before the schedule departure of train. No refund on
confirmed Tatkal tickets when cancelled within 24 hrs.of the schedule departure of train. For contingent cancellation
and waitlisted Tatkal ticket cancellations, charges will be deducted as per existing Railway rules.
Cancellation of e-tickets in case of trains cancelled:
If the train is marked as "CANCELLED" in PRS due to breaches, floods, accidents etc. full refund is permissible in
case the ticket is cancelled within three days of the scheduled departure of the train. In case of e-tickets, such
cancellations can be done by the customer through Internet.
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Good wishes