After I wrote my last post, the couple opposite were mercifully interrupted by a child throwing a bottle of water everywhere. Normally that would have annoyed me as well, but on this occasion it was the lesser of two evils, as they say.
Anyway, this afforded me the opportunity to see if I was right, and before I got off the train I asked if they were from Austria (I had nothing to lose, after all). I was wrong, they were from Germany. Bavaria, to be precise, which does have it's own strong dialect, but even so. *JP hangs his head in shame at his inability to accurately detect Germanic accents*.