The is a commuter electric multiple unit (EMU) train type operated by the private railway operator Seibu Railway in Japan since March 2017. a total of 20 ten-car trainsets and 3 eight-car trainsets have been built by Kawasaki Heavy Industries in Kobe from 2016, with the first trains entering service on 25 March 2017.
The new 40000 series trains were built by Kawasaki Heavy Industries in Kobe from January 2016, with deliveries scheduled to continue from fiscal 2016 to 2019. This is the first time that trains for Seibu Railway have been built by this manufacturer.
The new trains are designed as an evolution of the 30000 series "Smile Train" concept, and are intended to be passenger-friendly.
The 40000 series trains operate on Seibu Ikebukuro Line, Seibu Shinjuku Line, and Seibu Haijima Line services. Some trainsets operate on reserved-seat supplementary-fare S-Train services between the Seibu Ikebukuro Line and Tokyo Metro Yurakucho Line on weekdays and between the Seibu Chichibu Line and Minatomirai Line (via the Tokyo Metro Fukutoshin Line and Tà Âkyà « Tà Âyoko Line) at weekends since 25 March 2017.
The 40000 series EMUs are ten-car sets formed as follows, with five motored ("M") cars and five non-powered trailer ("T") cars, and car 1 at the Hanno (i.e. northern) end.
Cars 2, 5, and 8 each have one single-arm pantograph. Car 4 has a universal access toilet.
Due to the use of fixed longitudinal seating throughout, the 40050 subseries EMUs are slightly lighter and accommodate more passengers than the 40000 series trains.
Cars 2, 5, and 8 each have one single-arm pantograph.
The trains' interiors use LED lighting, and pairs of LCD passenger information screens are provided both above the doorways and suspended form the ceilings. A "Partner zone" area is provided at the end of car 10 with perch seats and space for wheelchairs and large luggage. This area also has larger windows. "Plasmacluster" air-purification technology is used for the first time on Seibu Railway trains.
Passenger accommodation consists primarily of rotating pairs of seats can be arranged in either longitudinal or transverse arrangements, known as "LONG/CROSS" seats.
Passenger accommodation consists of fixed longitudinal bench seating throughout. These seats are referred to as "LONG" seats.
Details of the new trains were officially announced in August 2015. The first trainset, 40101, was delivered from the Kawasaki Heavy Industries factory in Kobe to Seibu's Kotesashi Depot in September 2016.
The first sets entered revenue service on S-Train services on 25 March 2017.
From spring 2018, 40000 series trains are scheduled to be used on new limited-stop supplementary-fare commuter services operating from to on weekday evenings.
In 2019, Seibu announced plans to procure two new 40000 series trainsets. These sets, numbered 40151 and 40152, were built with fixed longitudinal bench seating throughout instead of rotating seats found in older sets. Two additional sets with fixed longitudinal seating were announced in 2020 as part of the operator's investment plan for fiscal 2020, followed by another three for 2021. By November 2023, fourteen 40000 series sets with fixed longitudinal seating had been built.
In May 2024, Seibu announced plans to build three new eight-car sets as part of its fiscal 2024 capital investment plan. The first eight-car set, 48151, was delivered in December 2024.
In February 2026, Seibu announced that it would introduce 8-car trains based on the 40000 series design, branded , to replace the 10000 series fleet on Shinjuku Line limited-express services. Four sets are scheduled to be introduced by the first quarter of 2027. Internally, the trains will feature rotating pairs of reclining seats throughout, and every seat will be equipped with power outlets, and cup holders. Seibu estimated that the new trains would be 17% lighter and offer 70% energy savings over the 10000 series.
The manufacturers and delivery dates for the fleet are as shown below.