Replacing the E231-500 series, the EMU JR East E235 series has recently become the large force on the Yamanote line.
The E235 series was launched on November 30th, 2015 as the latest model on the Yamanote line for the first time in 13 years. It is an 11-car direct current EMU consisting of 6 motor cars and 5 trailers. As of January 2018, a total of 14 sets (151 units) are operated. Most of them were newly built by J-TREC Niitsu Factory, but 12 units (the Type SaHa E235) were modified from the Type SaHa E231-4600 of the EMU E231 series.
The Yamanote line is well known as Japan's busiest railway. This line is looped through the downtown area of Tokyo. Its total route length is 34 km. The train covers the entire route in about 60 minutes both clockwise and counter-clockwise. There are 29 stations and most of them are interchange stations to the subway, private railways and/or other JR lines.
The E235 series has an advanced apparatus such as an SIC-element VVVF inverter (variable frequency drive) electric control system. I agree with its innovativeness; however, the outer design of the E235 series is not my cup of tea, to be honest. I call it "a microwave oven" because of its front design.
JR East plans to introduce a total of 50 sets of the E235 series by March 2020. It means that the EMU 231-500 series will be totally replaced by the E235 series by that time.
Official information about the EMU JR East E235 series (in Japanese):