Ammi Jaan kahti thi Railfanning se bada koi dharm nhi hota or koi Railfan chhota nhi hota - Shanzil Kabir

If you have an e-ticket:
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1) Yes, undoubtedly you can.
2) Log in to IRCTC,
Go under "Cancel ticket"
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confirm your password
Select ticket
Click on "Go For Cancellation" (no don't worry, this won't cancel the whole of it, we all have our hearts in our mouths when we click this for the first time)
You'd be redirected to a page where u can select for which all passengers you want to go ahead with the cancellation. Select them.
Click Cancel.
Confirm it again in the dialog box (Okay this one will actually cancel it, don't mess it up, its irreversible).
You're done.

If you have a counter PRS ticket:
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Fill up the form exactly the way you filled it for booking it.
Mention only those names for whom you want the tickets to be cancelled.
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at the top: For Cancellation
Give it to the clerk.
He'd do the rest and will give you the refund. (Don't expect him to give you exact change).