15 K V AC
The 15 kV, 16.7 Hz AC railway electrification system is used in Germany, Austria, Switzerland, Sweden and Norway. The high voltage enables high power transmission with low losses powering traction motors available since the beginning of the 20th century. Railway electrification in late 20th century tended to use 25 kV, 50 Hz AC systems which has become the preferred standard for new railway electrifications but extensions of the existing 15 kV networks are not completely unlikely.
Due to high conversion costs and little advantages it is unlikely that existing 15 kV, 16.7 Hz systems will be converted to 25 kV, 50 Hz.
Read more about 15 K V AC: History, Facilities For 15 KV AC Railway Electrification in Germany, Austria and Switzerland, Norway, Sweden