Green Line (Yokohama)

The line is the first completed piece of a much larger scheme, the Yokohama Circular Railway (横浜環状鉄道), conceived to ring the city's suburbs and tie its outer wards into the existing network. Its planning roots reach back to 11 July 1985, when the Transport Policy Council's Report No. 7 recommended a Yokohama Line 4 running from Hiyoshi via Takatachō and the Kōhoku New Town toward the Yokohama Line. The route was intended to link two of the city's most important rail hubs that the older Blue Line did not directly join — Hiyoshi, a major Tōkyū interchange in the north-east, and Nakayama on JR's Yokohama Line in the west — while opening up the rapidly developing New Town districts in between.

Formal authorisation followed in the late 1990s. The Transportation Bureau applied for a Type 1 railway business licence for the Hiyoshi–Nakayama Line 4 on 31 March 1997, and the licence was granted on 23 May 1997. Construction was timed to follow the methodical, building-block approach the city had used for its first subway, and a groundbreaking ceremony for Line 4 was held on 30 January 2001 at Center Minami Station, an existing Blue Line stop that the new line would cross. Building the route as a compact linear-motor metro kept tunnelling costs and cross-sections down on a corridor that climbs through hilly terrain.

The line's identity was settled while it was still under construction. On 15 June 2006 the Transportation Bureau decided to call Line 4 the "Green Line," the same day it chose "Blue Line" as the combined name for its existing Lines 1 and 3. The pairing was deliberate: when the new route opened, the city would have two colour-named subways to tell apart, and the established subway formally took up the "Blue Line" name on the very day the Green Line entered service.

The Green Line opened on 30 March 2008, running the full 13.0 km between Nakayama and Hiyoshi with ten stations from the outset. It became Japan's newest iron-wheel linear-motor subway of its generation and gave the Kōhoku New Town a fast cross-town link it had previously lacked, connecting at its ends to the Tōkyū Tōyoko and Meguro lines at Hiyoshi and to the JR Yokohama Line at Nakayama, and crossing the Blue Line at Center Kita and Center Minami. Although most of the route is in tunnel, the line runs in the open on a viaduct of about 2.4 km between Center Kita and Center Minami and surfaces again near Kawawachō and Tsuzuki-Fureai-no-Oka stations.

Services have been worked from the opening by the Transportation Bureau's purpose-built 10000 series, a linear-motor electric multiple unit. Sixty cars — fifteen four-car sets — were delivered for the start of operations in 2008, and a further eight cars (two four-car sets) were added in 2014 as ridership grew. To raise capacity further, the bureau began running six-car trains on 24 March 2022, with a programme to lengthen ten of the sets from four to six cars in stages.

The Green Line as built is only the beginning of the Yokohama Circular Railway plan, and the city continues to study extensions at both ends. From Hiyoshi the line is envisaged to run on toward Tsurumi, and from Nakayama it would push south and east via Futamatagawa, Tsurugamine, Higashi-Totsuka, Kami-Ōoka and Negishi, potentially reaching Motomachi-Chūkagai on the Minatomirai Line and so closing the loop. The Transportation Bureau classes these extensions as a long-term undertaking rather than an imminent project, so for now the Green Line remains the single Nakayama–Hiyoshi segment opened in 2008.

• 198511 July: the Transport Policy Council's Report No. 7 recommends a Yokohama Line 4 from Hiyoshi via Takatachō and the Kōhoku New Town toward the Yokohama Line.
• 199731 March: the Transportation Bureau applies for a Type 1 railway business licence for the Hiyoshi–Nakayama Line 4; the licence is granted on 23 May.
• 200130 January: a groundbreaking ceremony for Line 4 is held at Center Minami Station on the Blue Line; construction begins.
• 200615 June: the Transportation Bureau decides on the name 'Green Line' for Line 4 (deciding 'Blue Line' for Lines 1 and 3 the same day).
• 200830 March: the Green Line opens between Nakayama and Hiyoshi, 13.0 km with ten stations; the Blue Line name comes into formal use the same day.
• 2008Revenue services start with the purpose-built 10000 series linear-motor EMU: sixty cars (fifteen four-car sets) delivered for the opening.
• 2014A further eight cars (two four-car sets) of the 10000 series are added as ridership grows.
• 202224 March: six-car operation begins, under a programme to lengthen ten of the sets from four to six cars.