The E231 is a series of commuter EMUs developed for JR East in the end of the nineties. Based on the series 209 and E217, it is being powered by three-phase AC traction motors, has four doors per car and side and some cars with transverse seating. Compared to its predecessors, it has a wider body. Each set consists of four to ten cars, originally there were also eleven-car sets. From 2004, two cars of some sets were exchanged for double-decker cars.