The Semmering Railway is one of the masterpieces of 19th century architecture and engineering, but you have to be very interested in technology and related fields (I'm not) to appreciate these aspects of the railway (the viaducts are impressive though). Instead, do a nice daytrip from Vienna (or Graz) and enjoy the beautiful landscape in the area. Every train from Vienna to points south (Graz, Villach, all cities in Italy, Croatia, Slovenia) takes the Semmering pass route (at least until the construction of the railway tunnel, which is still in its planning stages), but you have to use the local trains with the destination "Mürzzuschlag" to be able to get off at the small town of Semmering at the top of the pass. There are numerous hiking trails (and skiing areas) there, as well as many hotels and pensions, many from the late 19th century, when the area was a favorite countryside retreat for wealthy Viennese.