Opened in 1932 together with the Kamimiyori ~ Yunokami section of the Aizu Line. Heading towards the #2 tunnel from Yunokami, the line went through a flat plateau of agricultural land of Idairako (location of the Saifukuji Buddhist temple), where the line made a turn to the east before continuing northwards again.
No. 2 Onumazaki Tunnel was quite short, about 100m in length, and the construction method was the same as for the rest of the Onumazaki tunnels, with the original tunnel wall being made entirely of concrete bricks, with a lining of poured concrete up until about halfway up the side walls. Later, the poured concrete layer on the lower half of the walls was given an extra layer, and a layer of wooden boards was applied to the arch of the ceiling where the brick was exposed; this was fastened in place by iron ribs that ran all the way around the arch from floor to floor.
トンネルは１９３２年で上三寄駅から湯野上駅までの路線部分と一緒に開かれた。湯野上駅から第二小沼崎トンネルに向かって走って線路は居平甲（西福寺の場所）での農地の平らな高原の中走った、そこにて線路は再び北さ回す前にしばらく東に向かって回した。
第二小沼崎トンネルは非常に短い、約１００mであった。残りの小沼崎トンネルたちのようにトンネルの元の壁はコンクリートレンガで作られた、壁の下半分にて注いたコンクリートの裏地があった。後ほど、コンクリートの裏地で追加の地層付け加えられた、と丸出しレンガは木の板で覆われていた。木の板は鉄の肋骨で開催された。
