It's not very direct, but a nice trip anyway.

Here's a possible itinerary:

08.18 from Wien Hbf

10.25 arriving Leoben Hbf

10.30 leaving Leoben Hbf

12.48 arriving Bischofshofen

13.07 leaving Bischofshofen 

13.48 arriving Salzburg Hbf

If you do hold a 1st class pass, there's a nice Swiss panoramic carriage on the 2nd train from Leoben (reservation recommended) and a restaurant car on the 1st train to have breakfast.

No worry about the 5 minutes change in Leoben, the connecting train is following on the same track.

In fact it's possible to do this that way every 2nd hour, but these are the nicest trains (just my 2 cents) (no railjet high speed trains).

It's also possible to go via Klagenfurt which would take two hours more but uses the famous Tauern railway. 

To verify timetables, use: https://fahrplan.oebb.at/webapp/

Thanks Hektor.  Is that two ‘journeys’ on my rail planner … Wien hbf to Leoben and Leoben to Salzburg. I’m new to Eurail and training around Europe so it’s all a bit daunting!! Hopefully all will become clear after the first journey. 

It works by pass days : on a day (00:00 - 23:59) you can take as many trains as you'd like, whether it's 1, 2 or 20 :)

There are regional (RE) trains on the section between Muerzzuschlag and Payerback-Reichenau, so definitely there are more trains than the high-speed ones only. Wikipedia tells me that the speed on that line is 140km/h so it is not that high speed. 

Perfect! Thank you all for your suggestions and useful info. I’ve downloaded to OBB Scotty App and can see the timetable. 

All trains go the up the Semmering pas slowly, as it is very curvy for a long climb. I think the local speed limit is 70 km/u or less. Check on openrailwaymap.org (there is a modus with speed limits). So it does not really matter wich type of train you take.

I Just checked openrailwaymap.org and there max speeds as low as 40km/h. Once the tunnel opens this wil be gone unless they keep the route for local trains, (7 stations). Only at Semmering do the faster trains also stop. There are popular hiking trains along the route to see the trains.

Thank you Marc