E217系は横須賀線・総武線快速用に設計されました。100 km以上の距離を走るものの、209系などの通勤電車と同じ数の乗降扉があります。東京駅付近の東京トンネルを通るため、地下区間の運転で必要な前面非常扉を備えています。のちに数編成が東海道線へ転用されました。各編成には2両の2階建てグリーン車があり、追加料金でより快適な設備を利用できます。
Series E217 was designed for the Yokosuka/Sobu Line rapid service. Although running more than 100 km, it has as many doors as commuter trains such as Series 209. Since it passes through the Tokyo tunnel near Tokyo Station, it has a front-end emergency door as required for underground operation. Several trainsets were later transferred to the Tokaido Line. Every trainset contains two double-decker Green Cars, which offer better accommodation for an additional charge.