The world's first true mountain railway, finished in 1854 and now UNESCO-listed, curls over the Semmering Pass on a parade of stone viaducts that look borrowed from a Roman aqueduct catalogue. The genius is accessibility: it's a normal line, so any regional train from Vienna to Mürzzuschlag rides it for a few euros — no reservations, no panoramic-carriage surcharge. Sit on the left leaving Vienna, and walk the Bahnwanderweg trail alongside for the viaduct views.
Featured in Ten mountain railways with views you won't believe · Mountain