The is a Japanese railway line connecting Toyohashi Station in Toyohashi, Aichi with Tatsuno Station in Tatsuno, Nagano, operated by Central Japan Railway Company (JR Central). The line links eastern Aichi Prefecture and southern Nagano Prefecture through northwestern Shizuoka Prefecture. It goes through steep mountains as well as cities such as Iida and Ina. The line was originally of four different private railway lines, the first of which opened in 1897.
The line has an unusually high number of so-called Hikyà  stations, or hikyo-eki, which have since lost their nearby communities due to depopulation. There are 94 total stations along the route. A number of these stations have become Hikyà  stations over the years. The line has been described as the "holy land for those who love touring hikyo-eki". The phrase was coined in 1999 by Takanobu Ushiyama and friends, for railroad stations that are isolated and little used.
For a distance of between and Tatsuno the Iida Line does not intersect with any other railway line.
, the following train services operate on the Iida Line:
The limited express operates two round trips per day between Toyohashi and Iida. The rapid runs one round trip per day between Iida, and (operating a through service on the Chà «à  Main Line (Tatsuno Branch), Chà «à  Main Line, Shinonoi Line & Shin'etsu Main Line).
Local service frequency changes multiple times along the line. From Toyohashi a 15-minute frequency operates as far as Toyokawa, approximately half-hourly as far as and hourly as far as . Between Hon-Nagashino and Tenryà «kyà  (a mountainous area with few passengers) services operate every 1-3 hours. North of Tenryà «kyà  services generally operate approximately hourly, with some sections having 30-minute frequencies in peak times. At Tatsuno, almost all trains continue to/from Okaya on the Chà «à  Main Line (Tatsuno Branch).
Traveling the entire length of the line by local train takes over six hours and generally requires changing trains one or more times. There are only two northbound and three southbound trains per day that operate the entire length of the line between Toyohashi and Tatsuno, with all of them continuing to/from Okaya (one of the southbound trains commences at ).
As of 2025, the Iida Line hosts the longest local train service in Japan - the 14:38 train from Toyohashi arrives at Okaya at 21:37 with a running time of 6 hours 59 minutes (419 minutes) for the 96-station journey.
All trains stop at stations marked "âÂÂ" and pass stations marked "-", "âÂÂ", or "âÂÂ". Arrows indicate the direction of rapid trains. Some trains stop at stops marked "â²".
All trains stop at stations marked "âÂÂ" and pass stations marked "-", "âÂÂ", or "âÂÂ". Arrows also indicate the directions the rapid trains run.
The Toyokawa Railway opened the section from Toyohashi to Toyokawa in 1897, extending the line to Omi in 1900. At the northern end, the Ina Electric Railway opened the Tatsuno to Ina-Matsushima section (electrified at 1,200 V DC) in 1909, extending the line to Tenryukyo in sections between 1911 and 1927. The Horaitera Railway opened the Omi to Mikawa-Kawai section in 1923, and electrified it at 1,500 V DC in 1925 in conjunction with the electrification of the Toyohashi to Omi section the same year. The Toyohashi to Toyokawa section was double-tracked the following year. The Sanshin Railway opened the Tenryukyo to Kadoshima section as an electrified (1,500 V DC) line in 1932, the Mikawa-Kawai to Toei section (and all subsequent stages) as an electrified line in 1933, and connected the two sections (completing the line) in 1936. All four companies were nationalised in 1943.
In 1955, the overhead line voltage of the Tatsuno to Tenryukyo section was increased to 1,500 V DC.
CTC signalling was commissioned on the line between 1983 and 1984, and freight services ceased in 1996.