The E217 series Electric Multiple Unit was the first outer-suburban train type with four doors per side on car. It was introduced by JR East in 1994 to replace the 113 series EMUs running on the Sōbu Line and Yokosuka Lines.

In March 2006, three 15-car trains were reformed as 10+5-car sets and transferred to the Kōzu depot for use on the Tōkaidō Main Line out of Tokyo alongside E231 series trains. The sets were repainted into the same Shōnan color scheme of orange and green stripes as applied to the E231s.