[Update, May 20] The new rules start today, and there have been some slight changes since this article was written. If you're travelling with luggage with a total length, height and depth measurement of more than 160cm, you'll have to reserve an 'oversized luggage space seat', which includes special storage space for your baggage behind the seat. It's usually in the last row inside a shinkansen car. You can't take and store your oversized luggage in the non-reserved seat cars. Strollers, bicycles and musical instruments are exempted and don't need a special reservation.
Japan is famous for its shinkansen. These bullet trains are fast, comfortable and punctual – just the most enjoyable and efficient way to travel across Japan, especially if special discounts are available to overseas tourists. While currently there are no restrictions on luggage carried on board, this is about to change next year.
Starting May 2020, you are required to make a prior reservation for large suitcases that exceed a certain size. This is a new initiative to promote a safer journey due the influx of tourists for the upcoming Olympics and Paralympics. Reservations can be made online or on the spot at the shinkansen station service counter for free. Your baggage will then be stored in a locked compartment of the train for your journey.
This regulation affects the following shinkansen lines: Tokaido Shinkansen that runs from Tokyo to Shin-Osaka via Kyoto; Sanyo Shinkansen between Shin-Osaka and Hakata, with stops in Shin-Kobe, Himeji and Hiroshima; as well as the Kyushu Shinkansen from Hakata to Kagoshima-Chuo. Bags with a total length, height and depth measurement exceeding 160cm (up to a maximum of 250cm) will require a reservation. Failure to do so before boarding will incur a ¥1,000 penalty. To make things easier – ie, so you know if you should make the luggage reservation – measuring devices for suitcases will be installed near the shinkansen gates at all the stations along the affected routes.