In spring season, the busiest period is late April and the beginning of May. It is called “Golden Week” in Japan. April 29, May 3, 4, and 5 are National holidays. If you can, lease try to avoid travelling on those days.
Kushiro Shitsugen (Kushiro marsh) is national park and has the largest marsh wetland in Japan. It is well known as the home of endangered Japanese Cranes. JR Hokkaido’s Senmo line goes through this wetland and gives you a great scenery. Kushiro Shitsugen Norokko train is operated on this line from late April to late September. This is the easiest way to enjoy a day excursion in Kushiro Shitsugen from Kushiro. And Kushiro can be reached by Limited Express Super Ozora from Sapporo easily. Both Super Ozora and Kushiro Shitsugen Norokko train are fully covered by Japan Rail Pass.