15kVAC
From Freepedia
15kVAC albeit at 16.7Hz is a railway electrification system chosen by Germany, Austria, Switzerland, Sweden and Norway in 1912. The high voltage enables high power, while the lower voltage reflects technical limitations at the turn on the 20th Century. In particular, the lower voltage reduced flashover problems in the motors, albeit at the cost of a non-standard line frequency requiring frequency conversion, and separate supplies.
This voltage has been superseded by 25kVAC at 50Hz or 60Hz since the 1950s.
Oddly, when Denmark decided to electrify in the 1970s they chose not to use 15kVAC which would have linked compatible systems in Germany and Sweden, but rather chose 25kVAC.
