Tohoku Shinkansen runs in the east half of Tohoku area. It has two branch lines to the west Tohoku area: Akita Shinkanasen and Yamagata Shinkansen.
To reduce the construction cost, the two branch lines were converted from existing lines of JR. The problem with the conversion is that Shinkansen uses the standard gauge while narrow gauge is used by the existing lines. JR changed the gauge, but did not modify the stations. So, a normal Shinkansen train is too large to enter the stations of Yamagata and Akita Shinkansen.
E3 series was designed for Akita and Yamagata Shinkansen. It is smaller than normal Shinkansen trains and sometimes called Mini-Shinkansen.
The train in the picture is Akita Shinkansen E3 series train. Its nickname is "Komachi". It runs with E2 or E4 series train between Tokyo and Morioka. It runs separately on the branch line between Morioka and Akita.