that was a quick answer. It is indeed the transmission for the train lines. Andy and Paul are right about the reason - the frequency is different from that of the ordinary power grid.
I think your source is wrong. Denmark uses 50Hz, 25kV for the trains whereas Sweden and Norway use 16 2/3Hz, 15kV. This has forced trains that run between the countries to be dual voltage / dual frequency.
[This message has been edited by C-H (edited 10-19-2003).]