Agatsuma Line

The line began not as a passenger route but as a freight line. On 2 January 1945, in the closing months of the Pacific War, the 42.4-kilometre section between Shibukawa and Naganohara opened as the freight-only Naganohara Line. Passenger service followed in stages over the next year and a half as the route was extended station by station: services reached Nakanojō on 5 August 1945, Iwashima on 20 November 1945, and Naganohara on 20 April 1946.

For the next two decades the line remained an unelectrified rural branch. Electrification of the Shibukawa–Naganohara section at 1,500 V DC was completed on 10 June 1967, modernising operations along the existing route.

The line took its modern form on 7 March 1971. On that day the route was extended a further 13.3 kilometres beyond Naganohara to Ōmae, and the whole line was renamed from the Naganohara Line to the Agatsuma Line, after the Agatsuma River whose valley it follows. Centralised traffic control (CTC) was commissioned across the entire line at the same time.

With the privatisation and break-up of Japanese National Railways on 1 April 1987, ownership of the Agatsuma Line passed to the newly formed JR East, under which it has operated ever since.

The most significant change to the route came in 2014, driven by the construction of the Yanba Dam on the Agatsuma River. The reservoir created by the dam was set to submerge part of the existing alignment, so a replacement line was built on higher ground. The last scheduled trains ran over the old route on 24 September 2014, and the new alignment between Iwashima and Naganohara-Kusatsuguchi opened to traffic on 1 October 2014. The realigned section was 0.3 kilometres shorter than the route it replaced, reducing the line's total operating length to 55.3 kilometres.

The rerouting also removed one of the line's curiosities. The old alignment had included the Tarusawa Tunnel, just 7.2 metres long and long celebrated as the shortest railway tunnel in Japan; with the 2014 switch to the new line, the old route and its tiny tunnel were taken out of service and lost beneath the rising waters of the Yanba Dam reservoir.

• 19452 January: the 42.4 km Shibukawa–Naganohara section opens as the freight-only Naganohara Line.
• 19455 August: passenger service begins, reaching Nakanojō.
• 194520 November: passenger service is extended to Iwashima.
• 194620 April: passenger service is extended to Naganohara (later Naganohara-Kusatsuguchi).
• 196710 June: the Shibukawa–Naganohara section is electrified at 1,500 V DC.
• 19717 March: the line is extended 13.3 km from Naganohara to Ōmae, renamed from the Naganohara Line to the Agatsuma Line, and CTC is commissioned across the whole line.
• 19871 April: with the privatisation of Japanese National Railways, the Agatsuma Line comes under the ownership of JR East.
• 201424 September: the last scheduled trains run over the old alignment before the Yanba Dam rerouting.
• 20141 October: the new Iwashima–Naganohara-Kusatsuguchi alignment opens for the Yanba Dam construction; it is 0.3 km shorter, reducing the line to 55.3 km, and the old route's Tarusawa Tunnel (7.2 m, formerly Japan's shortest rail tunnel) is taken out of service and submerged.