Scenic narrow gauge trains in Austria

Roughly from west to east:

Bregenzerwaldbahn (museum railway; Bezau - Schwarzenberg)
Zillertalbahn (Jenbach - Mayrhofen)
Achenseebahn (Jenbach - Seespitz; steam cog railway!)
Pinzgauer Lokalbahn (Mittersil - Krimml)
Schafbergbahn, Salzburg (cog railway, some trains driven by steam, some by diesel engines)
Murtalbahn (Tamsweg - Mauterndorf, museum railway)
Traunseebahn (Vorchdorf-Eggernberg - Gmunden)
Mariazellerbahn (St. Pölten - Mariazell)

There are some more (which I haven't ridden yet); googling for "Schmalspurbahnen Österreich" may bring up additional results.

This got me thinking of my own plans. Here is an article that might be of interest
https://www.tripsavvy.com/beautiful-scenic-train-routes-in-austria-1508938
seat61.com has a short blurb for Semmering route and has a recommendation of how to get to Hallstatt from Vienna via this route.

I am pretty sure that you will stay in Vienna for a few days. There are three good options for a daytrip.

Waldviertelbahn
https://www.waldviertelbahn.at/en
Reachable via regional train from "Wien Franz-Josef-Bahnhof" station. Vintage steam engines at certain dates.

Schneebergbahn
https://www.schneebergbahn.at/en
Reachable via express train from "Wien Hauptbahnhof" and regional train from "Wr. Neustadt". Vintage steam engines at certain dates. Cog railway leading to Austria's highest train station.

Mariazellerbahn
https://www.mariazellerbahn.at/en
Reachable via express train from "Wien Hauptbahnhof" or "Wien Westbahnhof".

Thank you to everyone for the suggestions

If you are an avid trainspotter , this travelogue from 1980 ( a favorite of mine since then ) will enchant you , There is a section on the Zillertalbahn in it . Enjoy Q - https://youtu.be/hlZaicAXnFA