***Twilight Express was discontinued in 12 March, 2015. There is no overnight train option between Osaka and Hokkaido any more.***
Twilight Express is operated between Osaka and Sapporo. This is the longest service operation train in Japan. It takes more than 22 hours to reach Sapporo from Osaka. (If you take other way, it takes 22 hours 45 minutes.)
When you use a berth or a bedroom, Japan Rail Pass does not cover any accommodation charges and even Limited Express surcharge either. Even though you have to pay lots of the extra on Japan Rail Pass, the experience on board will be worthy. First class hospitality, the beautiful scenery and the sunset on the Sea of Japan, will be very memorable moment in your trip.
In summer season, the busiest period is mid August. This period is called “Obon (お盆)”. Everybody try to go back to hometown in this period. In this year 2012, August 10 to 19 will be Obon period. Especially the direction to go out from major cities on Aug 10-12 and the direction to go into major cities on Aug 17-19 will be much busier. These dates are weekend days. Everybody will try to move on these dates. If you travel in this period, try to go opposite direction. It makes you to travel much easier.
Some part of winter season is the busiest period. Especially in Japan, New Years holidays are crazy busy. In this year, December 30, 11 (Fri) and January 4, 12 (Sun) will be busiest days. Please try to avoid taking a train on those days. the vehicle occupancy rate may be more than 200%. Sometimes it is almost impossible to get in or out from the train.
Even though Japan is very small country, Japan have several climate zone. It is icy freeeeeezing weather in Hokkaido. South part of Kyushu have very mild weather. It is very interesting season to travel. But if you travel in Hokkaido, Tohoku or Hokuriku region, please expect the delay due to snow. I do not recommend you to make a tight schedule.