In Austria, there are rail lines on both sides of the Danube. The multi-
track mainline between Vienna and Salzburg is on the south side of the
Danube -- and probably far enough inland (at most locations) not to
be affected. (But of course it only takes one location to put a line
out of service. However, Austria has a dense enough rail network so
that in some cases detours on other lines -- though at slower speeds
and taking longer -- are possible.)

However, there is a secondary single-track line on the north side of the
Danube that connects lots of smaller towns and has branches diverging off
to the north. On that secondary line, the tracks are, at some locations,
only a few hundred feet from the Danube shore and only about a dozen feet
above the river.

I've been in many of the small towns on both sides of the Danube, though
it's been a number of years. The photo of Melk (the town with the huge
abbey above the town) is heartbreaking. This was always one of my favorite
towns in Austria.

I just looked at the (German language) site of the Austrian Federal Railways
(OBB) and they have a map showing where rail lines are disrupted:

http://fahrplan.oebb.at/bin/help.exe/dn?tpl=showmap_external

(I was born in Austria and came to the U.S. as a child with my parents. I
have made many trips to Austria since then, most of them primarily to ride
the trains there. I still speak and read German and have always enjoyed
talking with Austrian railroaders.)

-- Ernest