Which Austrian line, opened in 1854, is often called Europe's first mountain railway?

Which Austrian line, opened in 1854, is often called Europe's first mountain railway? The answer is the Semmering Railway.

The Semmering Railway crosses the Semmering Pass between Gloggnitz and Mürzzuschlag in Austria. Built between 1848 and 1854, it is widely regarded as Europe's first mountain railway constructed with standard-gauge track and is often described as the world's first true mountain railway.

Engineer Carl von Ghega designed the route to overcome steep gradients and difficult terrain using curves, tunnels, viaducts and substantial stone bridges. Around 20,000 workers took part in construction. The 41-kilometre line rises about 460 metres, with gradients reaching 2.5 percent and curves as tight as 190 metres in radius.

The railway also influenced locomotive design: special Engerth locomotives were developed to handle its gradients and curves. It remains part of Austria's Southern Railway and was added to the UNESCO World Heritage List in 1998. “First mountain railway” is a conventional historical description, not a claim that no earlier railway ever crossed hilly ground.