For a long time, I planned to focus on northern Tohoku. (I know it's not strictly necessary to pick a region, but it does help keep down the temptation to buy every interesting train.
) But through a combination of chance (where some trains we bought actually run) and my interest in both the Japan Sea coast and the mountains, I think I've settled on Niigata and adjacent portions of Gunma and Nagano, and maybe bits of Toyama, Yamagata, and Fukushima.
And since I'm at least as much of a map geek as a train geek, it helped me a lot to have
Kitayama's maps available online. I downloaded some of the maps and highlighted the lines I was thinking of modeling, or at least mimicking with the appropriate trains. I thought I'd post it here and see what people thought, and Göran has been kind enough to give permission for his maps to be used on the forum (with credit given).
On the map, the green highlighting shows the lines I'm thinking of (also listed at the top left), and the odd salmon color shows lines I'm not planning to model, but whose scenery I plan to check out for "inspiration." The green dots show two railways -- Nagano Electric and Shinano -- that I'm also thinking about.
