Zairaisen

Japanese railway lines can be categorized into two groups: Shinkansen lines, as defined in Article 2 of the Nationwide Shinkansen Railway Development Act (全国新幹線鉄道整備法) ("main railway lines on which trains can operate at speeds of 200 km/h or higher"), and other lines that do not meet this criterion.

The concept of zairaisen emerged with the opening of the Tōkaidō Shinkansen in October 1964, to distinguish the existing JNR lines from the newly introduced Shinkansen. Therefore, before the advent of the Shinkansen, the concept and definition of zairaisen did not exist. Since then, zairaisen has come to refer to railway lines in Japan, where trains operate at speeds of 160 km/h or less. ^[1]

Lines such as the Akita Shinkansen and Yamagata Shinkansen, commonly referred to as "Mini-shinkansen", are labeled as "Shinkansen" for passenger guidance. However, since their main sections do not allow operation at 200 km/h or higher, these lines are legally classified as zairaisen. ^[1] Also, even if they use Shinkansen-standard infrastructure and rolling stock, lines such as the Hakataminami Line and the Jōetsu Shinkansen branch line between Echigo-Yuzawa — Gala-Yuzawa are considered zairaisen in terms of passenger operations.

Under the Japanese National Railways Rehabilitation Special Measures Act (日本国有鉄道経営再建促進特別措置法) (JNR Reconstruction Act), zairaisen are further divided into main lines (幹線) and local lines (地方交通線), whereas all Shinkansen lines are classified as main lines. (The word "Shinkansen" is derived from shin (新; "new") + kansen (幹線; "main line").)

Generally, when Shinkansen services are introduced, passengers tend to shift from zairaisen limited express trains to the faster Shinkansen. As a result, most limited express services on zairaisen that run parallel to Shinkansen lines—except for night trains—are discontinued. However, in cases where a limited express serves areas not accessible by Shinkansen, it may continue to operate on sections that run parallel to the Shinkansen near major urban centers (e.g., Tokyo — Atami, Nagoya — Maibara — Shin-Osaka — Himeji, or Kokura — Hakata — Shin-Tosu).

The word zairaisen (zairai (在来; "conventional") + sen (線; "line")) did not exist before the Shinkansen era. When Japan's first Shinkansen, the Tōkaidō Shinkansen, opened on October 1, 1964, the term was coined as a retronym to distinguish traditional JNR railway lines—including those built later under conventional standards—from the newly introduced Shinkansen. Thus, zairaisen came to serve as an antonym to Shinkansen.

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Heikō zairaisen (並行在来線; "parallel conventional lines") refer to existing zairaisen lines that run alongside sections of the Seibi Shinkansen (整備新幹線; "Development Shinkansen"), planned or under-construction Shinkansen lines. ^[2][3]

Concerning the Seibi Shinkansen, a government-ruling party ^[a] agreement dated December 24, 1990, titled "Government and Ruling Party Agreement on the Start of Construction of the Seibi Shinkansen" (整備新幹線着工等についての政府・与党申合せ), stipulated that "for any segment approved for construction, the heikō zairaisen (parallel conventional line) must be confirmed to be separated from JR's management by the time of opening, prior to the approval of the construction". This was reaffirmed in another agreement ^[b] dated December 25, 1996, "Government and Ruling Party Agreement on the Handling of the Seibi Shinkansen" (整備新幹線の取扱いについて 政府与党合意) ^[4], which stated: "As previously agreed, the heikō zairaisen lines for the segments approved for construction shall be separated from JR's management upon the opening of the new line. The specific segments to be separated shall be finalized with the consent of the local governments along the route and JR, prior to the approval of the construction implementation plan for those segments".

Therefore, when the opening of a Seibi Shinkansen leads to the discontinuation of limited express and other services on zairaisen—making those lines unprofitable—JR is allowed to separate their management, except for sections it deems capable of generating sufficient revenue (as discussed later). This is intended to avoid the increased financial burden of operating such lines alongside the Shinkansen, and as a result, these sections are entrusted to third-sector (semi-public) railway companies. In some cases, such as the Yokokawa — Karuizawa section of the Shin'etsu Main Line, the line has been permanently closed.

This transfer of heikō zairaisen from JR to third-sector operators—carried out to reduce JR's financial burden—has sometimes led to significantly higher fares compared to when the lines were under JR management. Because the construction of the Seibi Shinkansen and the separation of heikō zairaisen are inseparable processes, the handling of such zairaisen has become a political issue, especially among local governments along the new Shinkansen routes. When transferring between JR and third-sector segments, discounted fares may be offered depending on the segment.

Even for heikō zairaisen, JR may choose to retain and operate segments that are expected to remain profitable or are necessary due to transportation system requirements. For example, the Kagoshima Main Line (Hakata — Yatsushiro and Sendai — Kagoshima-Chūō) parallel to the Kyushu Shinkansen, and the Shin'etsu Main Line (Takasaki — Yokokawa and Shinonoi — Nagano) parallel to the Hokuriku Shinkansen, fall under this category.

The Tsugaru Line section between Aomori — Shin-Naka-Oguni Signal Station [ja], which runs parallel to the Hokkaidō Shinkansen, is also retained and operated by JR. However, in this case, JR East is the operator of the Tsugaru Line, while JR Hokkaidō operates the Hokkaidō Shinkansen. Therefore, the Tsugaru Line is not officially classified as a heikō zairaisen. ^[5] Similarly, the Toyono — Iiyama section of the Iiyama Line, although geographically parallel to the Hokuriku Shinkansen, is not considered a heikō zairaisen because the alignment through Iiyama Station was incidental, not intentional. ^[c] As a result, JR East continues to operate this section even after the Shinkansen extension opened.

Some branch lines of JR that diverge from heikō zairaisen—such as the Hanawa Line, Iiyama Line, and Nanao Line—have all trains running to Shinkansen-served stations. After the opening of new Shinkansen lines, these services were revised to operate into third-sector segments due to the management separation. Except for the Nanao Line, only JR rolling stock is used, resulting in one-way through service.

When a newly constructed Shinkansen station is located away from the existing city center station, the management of access trains serving the new station becomes an issue. Of the already opened segments, JR continues to operate the Ōu Main Line segment between Shin-Aomori — Aomori, and the Hakodate Main Line segment between Shin-Hakodate-Hokuto — Hakodate. In the former case, JR East does not consider it a parallel line and therefore does not plan to separate its management. ^[d] In contrast, JR Hokkaidō regards the latter segment as part of the heikō zairaisen to be separated upon the future opening of the Shin-Hakodate-Hokuto — Sapporo segment of the Hokkaidō Shinkansen. For the Oshamambe — Yoichi section, local municipalities have agreed to a bus conversion, and the railway is scheduled to be discontinued once the Sapporo extension opens. ^[6]

However, the classification of the Shin-Hakodate-Hokuto — Hakodate section as a heikō zairaisen is disputed. The city of Hakodate, which maintains that it is not a heikō zairaisen, has requested that JR continue operations. ^[7] Since the final decision on whether a section is considered a parallel line and whether it should be separated lies de facto with each JR company, disputes with local governments have arisen across the country. ^[8]

In the case of the Nishi Kyūshū Shinkansen (Takeo-Onsen — Nagasaki), the management of the heikō zairaisen became controversial during negotiations with local governments opposed to the separation. As a result, an alternative approach involving vertical separation model [ja] (separation of infrastructure and operations) was considered. Ultimately, for the Kōhoku (formerly Hizen-Yamaguchi) — Isahaya section, JR Kyūshū continues operations under the vertical separation model for 23 years after the Shinkansen's initial opening. Meanwhile, another section (Hizen-Hama — Nagasaki) was converted to a non-electrified line following the removal of electrification equipment, marking a significant operational shift.