Kashii Line

The railway began not as a passenger line but as a colliery carrier. It was built by the Hakata Bay Railway (博多湾鉄道), which opened the section between Saitōzaki and Sue on 1 January 1904, establishing stations including Saitōzaki, Nata, Kashii, Doi, Chōjabaru and Sue. The purpose was to move coal from the Kasuya coalfield — in particular from a naval colliery at Shinbara — out to the port at Saitōzaki. In June 1904 a temporary line was pushed on from Sue to Shinbara so that the coal, which had until then been carted by road to Zassonokuma (today's Minami-Fukuoka), could be hauled directly by rail.

The line was then completed in stages. The Sue–Shinbara section opened formally on 1 June 1905, and full-scale shipment of warship coal from the naval Shinbara colliery began. On 29 December 1905 the final extension from Shinbara to Umi opened, completing the through line and establishing Umi Station. Freight branches followed: a goods branch from Sakadonoe to Shime opened in 1909, and was extended from Shime to Tabiishi — the later Tabiishi branch — in 1915, serving the collieries of the district.

On 25 March 1920 the company, having taken up a shipping business, changed its name from Hakata Bay Railway to Hakata Bay Railway Steamship (博多湾鉄道汽船). The wartime consolidation of Japan's transport industry then ended the company's independence. Under the policy that merged regional operators, Hakata Bay Railway Steamship was absorbed into the Nishi-Nippon Railroad (Nishitetsu) on 19 September 1942, and the route became that company's Kasuya Line together with the Tabiishi freight branch.

The line passed to the state during the Second World War. On 1 May 1944 it was bought up and nationalised, so that the Nishi-Nippon Railroad Kasuya Line became the Japanese Government Railways' Kashii Line, together with the Kashii Line freight branch (the Tabiishi branch); at the same time Nata Station was renamed Gannosu. Freight, once the line's whole reason for being, was wound down over the following decades as the Kasuya collieries closed: the Tabiishi branch was abolished in 1960, freight between Sakadonoe and Umi ceased in 1981, and the remaining goods services and the rest of the branch were discontinued by 1985.

With the division and privatisation of Japanese National Railways, the Kashii Line passed to the Kyushu Railway Company (JR Kyushu) on 1 April 1987. JR Kyushu modernised operations steadily: line-wide one-man (driver-only) operation began in 1988, and in 1989 the scenic Saitōzaki–Kashii section, which crosses the Umi-no-Nakamichi sandbar, was given the nickname "Umi-no-Nakamichi Line." From 2015 most of the intermediate stations were converted to unstaffed "Smart Support Station" operation, and in 2017 centralised traffic control (CTC) and automatic route control (PRC) were brought into service across the whole line. A blue line colour and JD station numbering were adopted in 2018.

The line's modern signature is its rolling stock. On 16 March 2019 the BEC819 "DENCHA" battery-electric multiple unit entered service on the Kashii Line, allowing electric-train operation on a route that has no continuous overhead wire — the trains charge from the electrified yard at Kashii. The non-electrified, self-contained line then became a testbed for automatic driving: running trials of automatic train operation with the BEC819 were carried out on the Saitōzaki–Kashii section over the winter of 2019–2020, demonstration running in revenue service began in December 2020, and from 16 March 2023 some services began running under automatic operation with an attendant on board rather than a fully qualified driver — an early example of higher-grade automation on a Japanese conventional line.

• 19041 January: the Hakata Bay Railway opens the Saitōzaki–Sue section, the line's first segment, to carry coal from the Kasuya coalfield to the port at Saitōzaki.
• 19051 June: the Sue–Shinbara section opens formally and full-scale shipment of warship coal from the naval Shinbara colliery begins.
• 190529 December: the final Shinbara–Umi extension opens, completing the through line and establishing Umi Station.
• 19091 August: a freight branch from Sakadonoe to Shime — the later Tabiishi branch — opens, serving the collieries of the district.
• 192025 March: on taking up a shipping business, the company is renamed from Hakata Bay Railway to Hakata Bay Railway Steamship.
• 194219 September: Hakata Bay Railway Steamship is absorbed into the Nishi-Nippon Railroad (Nishitetsu); the route becomes its Kasuya Line and the Tabiishi freight branch.
• 19441 May: the line is bought up and nationalised — the Nishi-Nippon Railroad Kasuya Line becomes the Japanese Government Railways' Kashii Line (with the Tabiishi freight branch); Nata Station is renamed Gannosu.
• 19851 January: freight is wound down with the closure of the Kasuya collieries — the Tabiishi branch having been abolished in 1960 and Sakadonoe–Umi freight in 1981, the remaining branch and the Kashii–Sakadonoe goods service are now discontinued, ending the line's freight traffic.
• 19871 April: with the division and privatisation of Japanese National Railways, the Kashii Line passes to the Kyushu Railway Company (JR Kyushu).
• 198813 March: line-wide one-man (driver-only) operation begins.
• 198915 March: the Saitōzaki–Kashii section, which crosses the Umi-no-Nakamichi sandbar, is given the nickname 'Umi-no-Nakamichi Line'.
• 201514 March: twelve stations (all except Chōjabaru and Kashii) are converted to unstaffed 'Smart Support Station' operation.
• 201729 January: centralised traffic control (CTC) and automatic route control (PRC) enter service across the whole line.
• 201916 March: the BEC819 'DENCHA' battery-electric multiple unit enters service, electrifying operation on a line with no continuous overhead wire; the trains charge from the yard at Kashii. (A blue line colour and JD station numbering had been adopted on 28 September 2018.)
• 202024 December: demonstration running of automatic train operation begins in revenue service, following running trials with the BEC819 over the winter of 2019–2020.
• 202316 March: some services begin running under automatic operation with an attendant on board rather than a fully qualified driver.