Nippon rails is a good challenge for experienced players. It is *very* constrained, and you always have to be careful where you build. There are 4 islands in Japan (Honshu - the main one, Hokkaido - the big one up north, Kyushu - south, and Kitakyushu - tucked between Honshu and Kyushu) and there are major cities on 3 of them, which means all players must be able to connect. However, there are only 5 ways to access Hokkaido, and 5 to access Kyushu (2 of which run through Kitakyushu which itself only has 3 accesses from Honshu). So fairly often someone will try to build, only to be told "You can't do that - you're blocking me."

Add in a mountainous terrain, a long, narrow board (the cheapest connection between any 2 major cities is Osaka-Tokyo along the coast for 29) and you have a challenging game. But fun. :)