[3] Construction of a base tunnel on the Yamagata Shinkansen is proposed, with JR East having undertaken a survey of a planned route from Niwasaka to Sekine, just south of Yonezawa station.

To be built on an improved alignment, the tunnel would lower journey times between Fukushima and Yamagata by ~10 min due to a proposed line speed of up to 200 km/h (124 mph).

The curvature and steep grades limit train speeds to 55 km/h (34 mph) or less, and the line is vulnerable to heavy rain and snowfall as well as high winds.

[4] Between 2011 and 2017 a total of 410 Yamagata mini-Shinkansen services were either suspended or delayed, and 40% of these incidents occurred on the line over the Itaya Toge pass.

[6] The cost could increase by ¥12 billion if the tunnel were to be built with a cross-section large enough to permit the line to be upgraded to the full Shinkansen Loading gauge.