The line opened on 2 January 1945 as the freight-only Naganohara Line operating between and Naganohara Station. Passenger services were introduced as far as from 5 August 1945, to from 20 November 1945, and to Naganohara-Kusatsuguchi from 20 April 1946. On 1 October 1952, the line was extended to Ōshi Station, initially for freight only, but passengers services were also introduced from 21 June 1954. Freight services between Shibukawa and Ōshi ceased as of 1 October 1966. Services on the section between Naganohara and Ōshi were suspended as of 1 November 1970, and on 7 March 1971, a new line was opened beyond Naganohara to, with the entire line renamed Agatsuma Line at the same time. The Naganohara-Kusatsuguchi to Ōshi line was formally closed as of 1 May 1971. CTC signalling on the entire line was also commissioned at the same time. With the privatization of JNR on 1 April 1987, the Agatsuma Line came under the ownership of JR East. It had been proposed to extend the line to Nagano, but geological exploratory drilling revealed extensive faulting beyond Ōmae, and as any extension would have required extensive tunnelling, this was considered impractical.
The Yanba Dam was proposed for construction, and required the realignment of the line between and. Construction of the diversion was well advanced when a change of government in 2009 resulted in the project being halted. Another change of government in 2012 revived the project. Services on the old section of the line were suspended following the last scheduled service on 24 September 2014 to allow commissioning of the new alignment. The new alignment opened for passenger services on 1 October 2014. The new route was be 0.3 km shorter, and resulted in the shortest tunnel in Japan, the 7m Tarusawa Tunnel, being abandoned, although it will not be inundated by the new dam.
Points of interest
The Agatsuma Line is noted for numerous onsenhot springs along the route. The famous hot springs at Kusatsu are some distance north of the line, but several rural onsen such as those at Shima, Sawatari, Kawarayu, and Shiriyaki are more accessible. Mount Asama, Mount Kusatsu-Shirane, and the Agatsuma Canyon can all be seen from the Agatsuma Line, though the canyon will be inundated when the Yanba Dam is commissioned.
