The section between Omiya Station and Saiin Station, which opened in 1931, is the second-oldest subway in Japan after the Tokyo Metro Ginza Line in Tokyo; but although it is the oldest in Japan as an overhead-line system, Sendai Station of the Miyagi Electric Railway (its location is different from that of the current Sendai Station on the JR Senseki Line) is the oldest in terms of the underground construction near the portion of the station in the overhead-line system.
なお1931年（昭和6年）開業の大宮駅～西院駅間は東京の東京地下鉄銀座線に次ぐ、日本で2番目の地下鉄である（架空電車線方式としては日本初であるが、架空電車線方式に限った駅部分付近の地下化に限っては宮城電気鉄道仙台駅（現在のJR仙石線仙台駅とは別位置)が先））。