I was just there in February and we did not run into this... I'm not talking about behind the seats, but in the luggage compartment by the doors. If that is the rule for this spot as well they did a terrible job of communicating that... there were no signs, announcements either over the intercom or on the LED signs at the front, nor did they mention it at all at the several JR Rail counters we visited! And we had all our luggage there with us... Is it maybe because we had reserved seats (regular, not oversize) so the luggage reservation was implied?