The ERT has 329 km for the current route Wien Hbf to Klagenfurt Hbf via Brück an der Mur and Unzmarkt.
It's going to change over the next decade, with several new constructions along the line.
The Semmering Pass section will be avoided by a new tunnel under construction, saving 14km or so (works at each end visible from the old line).
Once the Koralmbahn tunnel (between Graz and Klagenfurt) is finished the main route will be via Graz and a good deal shorter/quicker.
And I was reading that ÖBB plan to reroute fast services between Wien Neustadt and Meidling via Wampersdorf (the Pottendorfer Line).
This line is to be upgraded and doubled, freeing the main Südbahn route as a metro line.
This route would be about 7km longer than the present route.
The present route is highly scenic (I travelled Venice-Vienna this year), so it was good to experience it before the major changes kick in.