Nope its a technical halt. So you will have to book tickets upto or from Kulem and get down or board at DDS. But 12780 arrives DDS early at around 4am and DDS is not like other stations there are no facilities for passengers like a waiting hall, canteen, etc. So what many people do is book tickets till castle rock and start trekking from there early morning till the falls.
Regarding returning in 12779 it stops at DDS station but there will be a huge rush to board the train, so don't bring children or elderly people with you.
Last...
more... thing is be safe enjoy the environment there but stay away from swimming in the waters there during monsoons.