Note that taking overnight train will probably cost more than domestic air tickets between Tokyo and Hokkaido actually.
Even if you have JR Pass, but it only covers partial cost for the train ride, since you still need to pay for cabin charges and express charges and the partial non-JR tracks charges.
Cassopeia travels to Sapporo, and from Sapporo to Biei you have have to transfer to other trains.
For more info, including pictures, you can try going
to this website which helped me when I was planning for my overnight train journeys (Twilight Express, Sunrise Seto/Izumo, Akebono)