A total of 23 six-car sets are due to be introduced ahead of the 2025 World Expo,[4] and to replace the 20 and 24 series trainsets currently in use on the Chuo Line.

[3] The exterior design of the 400 series is intended to resemble that of a spaceship,[5] featuring an octagonal front end and headlight clusters at each of the four corners.

[3] The 400 series was developed in consideration of the 2025 World Expo, high levels of comfort and safety, accessibility, and to be "fun to ride".

[7] In addition, the train type has a lower floor height than its predecessors, as well as more clearly marked priority seating.

[5] Production of the 400 series fleet commenced[1] in 2022,[9] and the first set was delivered from Hitachi's Kasado plant in October of that year.