Thanks for your responses, that's very helpful.There seems to be some confusion from the first response. I am taking both routes.
sendai to tomakomai (depart on sept 18th)
tomakomai to orai (depart on sept 23rd)
So it looks like there are separate websites for those two routes?
I guess my one remaining question is how fast the reservations fill up, especially for first class I'm going to try calling them, but my Japanese isn't that great, so I'm wondering if I can just show up on the 18th and 23rd and there will be space available.