This app was mentioned in 8 comments, with an average of 3.13 upvotes
Zell am see is really popular in summer but don't let it put you off! It's a beautiful place. Hallstatt too. I'd go for the car rental as the you'd need multiple stops for the trains, and the trains are actually longer than car journeys as the trains take the long way round. Just as an example- it takes 1hour 10 mins to drive from sankt johann in tyrol to Salzburg but the train takes you about 2 and a half hours. Zell to Salzburg is about a 1.5hour drive or 2.15 hours by train.
If you're taking the train, download an app called OBB scotty. It's like a journey planner app for the trains.
You can use them for every public transportation in Austria
Your changing station is called Leoben. Your first train appears to be EC 163 which does not go through Germany and does not come close to Salzburg. There should be no problems with this train or the second one. For information on delays and timetable enquires, there is either the website or this application.
https://play.google.com/store/apps/details?id=de.hafas.android.oebb This seems to be the official App by the ÖBB.
You should try to give more information. Because depending on where you are, there might be shuttle bus services taking you directly to the RBR, from the hotel/hostel/place you're staying at. As someone else said already the other option is to take the train to Knittelfeld or Zeltweg Bahnhof and either walk the rest of the way or take the shuttle bus from there. I'd take the earliest train possible. I also suggest you download these two apps:
This app is basically a must have, if you're travelling by train. Shows routes, transfer times, delays, the platform the train arrives at. Basically everything.
ÖBB
This is the official ÖBB ticket app. If you don't manage to buy the leisure ticket, (which I suggest you do) you can buy and store tickets on your smartphone via this app.
Where did you look? The best app for public transport is Scotty imo