In 2026, whether you need a reservation depends on the specific line and your luggage size. Most Shinkansen trains offer both Reserved (Shiteiseki) and Non-Reserved (Jiyu-seki) cars. However, a major rule for 2026 is the Oversized Baggage Requirement on the Tokaido, Sanyo, and Kyushu lines. If the sum of your bag's dimensions is between 160 cm and 250 cm, you must reserve a "Seat with an Oversized Baggage Area" in advance. Failure to do so results in a 1,000 yen penalty and the conductor moving your bag. During peak travel periods like Golden Week, Obon, or New Year, nearly all trains become "Reserved Only," and getting a seat without a prior booking is virtually impossible. For a stress-free trip, especially on the popular Tokyo-to-Osaka route, booking a few days ahead via the Smart-EX app or at a ticket office is highly recommended to guarantee comfort.