I did a Smartbuy IRCTC transaction and made payment via HDFC Rupay Credit Card. Usually there is a 1.18% (1%+GST) charge on the transaction which is added when the transaction is settled. But it was not charged for the Rupay CC.
It's about a week now, I got the 5% cashback too.
So it is more beneficial if you pay on Smartbuy IRCTC via HDFC Rupay