Just be careful, technically this is not allowed. That space is only for people who booked last row seats.
Some conductors do not care, others will enforce the rules and ask you to move them - they can see on the tablet if it's been booked, obviously.
See FAQ below
From May 2020, passengers are required to reserve a seat with oversized baggage area when bringing baggage with overall dimensions of more than 160 cm/62 inches onto the Tokaido Shinkansen. This size is roughly the same as that for which an oversized baggage fee is charged on international flights.
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