[Rob Dyer]

Thanks Josh, hello @Quest!

You can choose a reserved seat in the last row and place your wheelchair behind the seat, provided your chair’s dimensions total no more than 250 cm (height + width + length limited to 2 meters), when folded down.

Here’s the official guidance on use of mobility scooters on trains in Japan (explains if you didn’t want to fold it down for travel):
https://www.mlit.go.jp/en/sogoseisaku/barrierfree/sosei_barrierfree_fr_000000.html

I suggest contacting one of the JR companies to inquire about this in more detail. Here’s a link to the accessibility page on the JR East site:

https://www.jreast.co.jp/e/customer_support/accessibility.html#category03

They have a telephone service helpline dedicated to non-Japanese customers:
https://www.jreast.co.jp/e/customer_support/infoline.html

There’s also this post about luggage on the shinkansen on the JRPass website – might be useful: https://click.jrpass.com/SH87

Have a great trip!