Apparently, Japan's big four copper mines were Hitachi (Ibaraki Pref.), Kozaka (Akita Pref.), Ashio (Tochigi Pref.), and Besshi (Ehime).

Hitachi city is day-trippable and roughly east from Nikko but there doesn't appear to be much to visit, apart from a museum, and I don't see a single track railway.
The Hitachi Mine

Looks like you'll have to do your prospecting from scratch next time you visit.