I was having a look at the link below (2 more 2-car 521 series sets delivered, these from Kinki Sharyo), and I got to wondering if these were dual current, or actually triple current. The venerable 485 series, for example, apparently runs on 1500VDC and 20kVAC at both 50Hz and 60Hz, while some other JNR-era equipment seems to be listed under one or the other.
I assume there has to be isolation in the catenary between two different frequencies, but must the train be switched when crossing from one zone to the other?
http://railf.jp/news/2015/02/07/191000.html
http://en.wikipedia.org/wiki/485_series