[2] The 20 series was the first Japanese train type powered by a third rail to use variable-frequency drive (VVVF) technology from new, the second overall in Japan (after the Kumamoto City Transportation Bureau 8200 series [ja] LRV and only the third such train type in the world (the HKL Class M100 for Helsinki Metro being the first overall).

The VVVF technology used on the 20 series was developed jointly by Hitachi, Mitsubishi Electric and Toshiba.

[4] The first set, 2601, was built on 25 March 1984, entering revenue service on the Chuo Line on 24 December of the same year.

[2] Four additional sets were built in 1985 to complement the opening of the Chuo Line extension to Nagata.

[1] From 2004, in preparation for the opening of the Keihanna Line extension to Gakken Nara-Tomigaoka in 2006, all 20 series sets were retrofitted with IGBT-based traction control systems and the maximum speed of all trains was increased from 70 km/h (43 mph) to 95 km/h (59 mph).