There are many little oddities in the Japanese railroad landscape and the Tatsuno Triangle, as I call it, is a good place to start documenting them. In southern Nagano Prefecture the Chuō Main Line [中央本線] run by JR East [JR東日本] takes a deep dive from Okaya Station [岡谷駅] following the Tenryū River to Tatsuno Station [辰野駅] before heading back north along the tighter Ono River valley to Shiojiri Station [塩尻駅]. This was the original 27.7km route between Okaya and Shiojiri when the line was established as mountains blocked a more direct route. However, in 1983 construction of the 6km long Enrei Tunnel [塩嶺トンネル] cut through those mountains directly connecting Okaya and Shiojiri creating the Tatsuno Triangle we see today.
Currently this is called the Tatsuno Spur Line [辰野支線] of the Chuō Main Line. All of the stations in the triangle are run and operated by JR East however Tatsuno and Shiojiri Stations both are the border between JR East and JR Central [JR中部]. Kawagishi Station [川岸駅] is the single station between Okaya and Tatsuno which appeared to be under construction when I passed it. About 20 trains a day that run on this section continuing down the Iida Line [飯田線] although a few do terminate at Tatsuno. Heading back from Tatsuno to Okaya most of the trains terminate at Okaya but at least one train a day will double back at Okaya and head through the Enrei Tunnel to terminate at Matsumoto Station [松本駅]. That would be a fun one to ride if you have the opportunity.
Heading from Tatsuno to Shiojiri there are only about 10 trains running north towards Shiojiri. All but two of these trains terminate at Shiojiri, with the first departure of the day heading all the way to Matsumoto and another one actually heading partially down he very rural Ōito Line [大糸線]. Heading back from Shiojii to Tatsuno there are also about 10 trains a day with everyone terminating at Tatsuno. Two stations, Ono [小野] and Shinano-Kawashima [信濃川島] are served on this line. The latter was quite bare, but the former had a nice building and some local character.
Completing the triangle is the newest section of track connecting Shiojiri to Okaya. Three to four trains pass through this section including limited express trains heading into the Kanto area and Tokyo. A single station named Midoriko exists just before the tunnel when heading towards Okaya. In this section the tracks are in a gully below the ground level so there is not much to see other than the platform and stairs to the entrance. While the name means “Green Lake” I was unable to see the lake it is referring to from the train.
I competed this triangle on a day trip while staying with friends in Ikeda, a village near Azumino just north of Matsumoto. In the morning I took the Ōito Line into Matsumoto. Originally my plan was to run the Shiojri-Tatsuno section first, as it was the least frequented. However, I was delayed at Matsumoto station and missed that train. This meant that I decided to ride the Shiojiri-Okaya section first and visit the onsen village at Shimo-suwa station then ride Okaya-Tatsuno-Shiojiri in the evening. This meant that most of the train riding occurred after dark, so I was not able to get a good view of the landscape. However, there were some lovely clouds that evening in the sky framed by the dark silhouette of the mountains. The cold early winter air had the smell of fresh water, trees and the smoke of a few heating fires burning in the houses we passed. I did not see many things in particular to point out along the line other than the last section from Ono coming into Shiojiri. This section hugs the hill on the west side of the train and the valley filled with houses and city lights spread out in front of you on the east side.
I did not exit the train at Okaya and only changed trains by walking across the platform at Tatsuno, so I do not have any comment about those stations. Shimo-suwa station, one stop down from Okaya, hosts a lovely hot spring village with some of the hottest baths I have ever experienced. I found my upper temperature limit with a 47˚C tub. Reviews of 4 day use hot spring facilities at Shimo-suwa station can be found on this site. Shiojiri Station is the biggest station in this triangle, and it hosts a thriving standing noodle bar, convenience store, local goods store and 2 restaurants in the station. The city is known for its grapes and wine so there is an actual grape vine growing on the platform in what they call the Platform Vineyard [ホームのぶどう園]. A 10 minute walk from the station the last remaining sento in Shiojiri city, Kuwa no Yu [桑の湯], which offers a simple, yet very refreshing, experience
Not the most beautiful train lines compared with what else you can see nearby but for the oddity factor the Tatsuno Triangle is a worthy route to take for any train enthusiast. By adding on some other local tourism, hot spring hopping for me, this became a very enjoyable day trip. There are also large Shintō temples and a very interesting clock museum among other attractions in Shimo-suwa to be explored if hot springs are not your thing.