What kind of waiting it was ? PQWL? GNWL ? RLWL ??
Anyway, the waitlisting tell you in terms and conditions that it can be changed depending on availability.
But you must write to Paytm with screenshots about it.
For booking train tickets, there is nothing better than IRCTC. Paytm has a lot of complaints, specially in booking bus tickets. I booked only once through Paytm, and they did not book "bedroll" for me, though I was charged. They refunded the charge later on, but there is always a screwup.
I think there is also an IRCTC app for reservations.