The 521 series is a dual-voltage AC/DC outer-suburban electric multiple unit (EMU) train type operated by West Japan Railway Company and third-sector railway operators IR Ishikawa Railway and Ainokaze Toyama Railway on local services in the Fukui and Kanazawa areas since November 2006. In March 2024, Hapi-line Fukui acquired some sets after taking over the section of the Hokuriku Main Line within Fukui Prefecture from JR West.

The IR Ishikawa Railway is a Japanese third-sector railway company established in 2012 to operate passenger railway services on the section of the JR West Hokuriku Main Line within Ishikawa Prefecture. The main line was separated from the JR West network on 14 March 2015, coinciding with the opening of the Hokuriku Shinkansen extension from Nagano to Kanazawa. It acquired another section from the JR West Hokuriku Main Line from Kanazawa to Daishoji on 16 March 2024 as the Hokuriku Shinkansen was extended from Kanazawa to Tsuruga. The company was founded on 28 August 2012 and has its headquarters in Kanazawa, Ishikawa.