Senzan Line

Proposals to connect Sendai and Yamagata by rail date back to the late Meiji period, but the choice of route across the mountains — over the Sekiyama, Futakuchi or Sasaya passes — long divided the two prefectures. The cities unified their lobbying efforts in 1919, and the line was written into the (revised) Railway Construction Act in 1922; construction was authorised in the same year. The Great Kantō Earthquake of 1923 delayed the start, and work on the Miyagi side did not begin until around 1926.

The railway was opened in stages from each end toward the mountains in the middle. On 29 September 1929 the Senzan East Line began operation between Sendai and Ayashi (15.2 km), opening Kita-Sendai, Rikuzen-Ochiai and Ayashi stations; this section was extended to Sakunami (13.5 km) on 30 August 1931. From the Yamagata end, the Senzan West Line opened between Uzen-Chitose and Yamadera (9.3 km) on 17 October 1933. Both end sections were initially worked by steam locomotives.

The two halves were joined on 10 November 1937, when the difficult mountain section between Sakunami and Yamadera (20.0 km) opened and the through route was renamed the Senzan Line. The heart of this section was the 5,361-metre Senzan Tunnel, at the time the third-longest railway tunnel in Japan after the Shimizu Tunnel on the Jōetsu Line and the Tanna Tunnel on the Tōkaidō Main Line. Because steam haulage was unsuitable for the long bore, the Sakunami–Yamadera segment was electrified from the outset at 1,500 V DC and worked by electric locomotives, while the outer sections continued under steam.

It was this electrified mountain section that made the Senzan Line nationally important. From October 1954 Japanese National Railways used the line as a test bed for commercial-frequency AC traction, beginning with trials between Rikuzen-Ochiai and Rikuzen-Shirasawa; in April 1955 the test section was widened to run from Kita-Sendai to Sakunami, and the experiments — Japan's first AC railway electrification — continued until April 1956. The results were decisive for the future of Japanese electrification: from 1957 the AC system was extended into Sendai Station, with AC-hauled freight running from July of that year, and purpose-built locomotives such as the ED45 (later ED91) were put into service.

Electrification of the rest of the line followed. The Yamadera–Uzen-Chitose section (continuing to Yamagata) was wired at 1,500 V DC on 1 November 1960, and around the same time the line gained named semi-express services such as the Asahi, Gassan and, from 1963, the Senzan. On 8 September 1968 the Sakunami–Uzen-Chitose section was converted to 20 kV 50 Hz AC, and immediately before the nationwide "Yonsantō" timetable revision of October 1968 the whole Senzan Line was unified under AC electrification.

Under the 1987 break-up and privatisation of Japanese National Railways, the Senzan Line passed on 1 April 1987 to JR East as the operating (Class 1) company, with JR Freight holding Class 2 freight rights. The opening of the Yamagata Shinkansen in the early 1990s briefly raised the line's profile, as Tsubasa and Tsugaru limited expresses were diverted over it during construction of the standard-gauge works, and the Sendai-end double-tracking to Uzen-Chitose was completed in 1999. Freight service ended in 2002. Today the Senzan Line is operated entirely with electric multiple units as a busy suburban and inter-city link; later 719-, 701- and E721-series trains have replaced the older stock, and the inner Sendai end forms part of the Sendai suburban network.

• 1922The Senzan Line is stipulated in the (revised) Railway Construction Act, and its construction is authorised.
• 192929 September: the Senzan East Line opens between Sendai and Ayashi (15.2 km); Kita-Sendai, Rikuzen-Ochiai and Ayashi stations open.
• 193130 August: the East Line is extended from Ayashi to Sakunami (13.5 km); Rikuzen-Shirasawa, Kumagane and Sakunami stations open.
• 193317 October: the Senzan West Line opens between Uzen-Chitose and Yamadera (9.3 km); Tateyama and Yamadera stations open.
• 193710 November: the Sakunami–Yamadera mountain section (20.0 km) opens, connecting Sendai and Uzen-Chitose; the through route is renamed the Senzan Line and the Sakunami–Yamadera section is electrified at 1,500 V DC. It includes the 5,361 m Senzan Tunnel, then Japan's third-longest after the Shimizu and Tanna tunnels.
• 1954October: Japanese National Railways begins AC electrification testing on the line, starting between Rikuzen-Ochiai and Rikuzen-Shirasawa — the first such trials in Japan.
• 1955August: AC (20 kV 50 Hz) electrification is energised on the test section around Sakunami; in April the test section had been widened to run Kita-Sendai–Sakunami.
• 1957AC electrification is extended to Sendai Station; AC-hauled freight begins in July, and the ED45 (later ED91) electric locomotive enters service.
• 19601 November: the Yamadera–Uzen-Chitose (–Yamagata) section is electrified at 1,500 V DC; the Asahi and Gassan semi-expresses begin.
• 19631 October: the Senzan semi-express begins operation between Sendai and Yamagata.
• 19688 September: the Sakunami–Uzen-Chitose (–Yamagata) section is converted to 20 kV 50 Hz AC, and the whole line is unified under AC electrification just before the October 'Yonsantō' timetable revision; the ED78 locomotive enters service.
• 198215 November: the Senzan express is downgraded to a rapid train and the Gassan express ceases operation on the Senzan Line.
• 19871 April: with the break-up and privatisation of Japanese National Railways, the line passes to JR East (Class 1) and JR Freight (Class 2).
• 199127 August: the Tsubasa limited express is diverted over the Senzan Line during construction of the Yamagata Shinkansen.
• 1999December: the line is double-tracked as far as Uzen-Chitose in conjunction with the expansion of Yamagata Shinkansen works.
• 20021 April: freight services cease on the Senzan Line.