Ive just booked a week in Horni Plana so have been checking up!
This year is the last year of operation by CD before the franchised operation takes over (GTR - no, not ours!). So do it while you can.
In summer, 2 out of 3 diagrams during the day are class 754 (Goggles) with 2-3 elderly coaches, at least one a double decker. The third is 2x842 and 2 trailers. I doubt this will change. From Cerny Kriz to Nove Udoli the frequency is hourly, the others come from Cicenice via Volary and are probably 814s.
The trip is well worth doing (uphill, window open!), but leave time for some walking/sightseeing.
There is also now a daily semi-fast from Prague and, in summer, an open-access working by Arriva as far as Nova Pec with an ex DB class 628.
Nove Udoli is in the middle of nowhere, but has a snack bar, a museum (?) and the aforementioned novelty railway. And at weekends a limited bus connection with the Ilztalbahn down to Passau.
Steam is most likely to operate in July or early August going by previous years.
Watch out for RRBs - the line was still being modernised last summer though I think it's finished.