It is commonly referred to as the world's first true mountain railway, given the very difficult terrain and the considerable altitude difference that was mastered during its construction. It is still fully functional as a part of the Southern Railway which is operated by the Austrian Federal Railways.
The Semmering railway was constructed between 1848 and 1854 by some 20,000 workers under the project's designer and director Carl von Ghega born in Venice as Carlo Ghega in an Albanian family. The construction features 14 tunnels (among them the 1,431 m summit tunnel), 16 viaducts (several two-storey) and over 100 stone arch bridges and 11 small iron bridges. The stations and the buildings for the supervisors were often built directly from the waste rock dug out when making the tunnels.
Across an overall track length of 41 km the Semmering railway overcomes an altitude difference of 460 m; on 60% of its length the gradient is 2.0-2.5% (equivalent to a 1-meter difference in altitude on a 40 m route distance) and 16% exhibit a curvature radius of only 190 m.
This was an entirely new technical dimension of railway construction, and new instruments and methods of surveying had to be developed to handle the resulting challenges. Also, new technologies were employed for the Engerth locomotives because the types in general use at this time could not handle the extreme gradients and turning radii.
In 1998 the Semmering railway was added to the list of the UNESCO World Heritage sites.
According to en.wikipedia