For the Mittenwaldbahn: please note that there are two versions existing - from RSSLO and from DTG. Both has their advantages and disadvantages.
Same for the Südbahn route. But here the DTG version only runs between Gloggnitz and Mürzzuschlag, the RSSLO version is much longer.
The newer routes (Karawankenbahn, Salzburg - Mühldorf, Salzburg - Linz, Salzburg - Saalfelden, Wörgl - Holzkirchen, Wörgl - Saalfelden has a better track laying (smoother) than the older ones, but the "Tirol Austria" or "Arlbergbahn" are nice too.
For the Tauernbahn I recommend to wait, because RSSLO have a update / new extension in planning