Rail Wars! S01 E06 I'll Protect You.

"All aboard for adventure, action, and a high-speed pursuit of dreams!"

The members of K4 have been punished with at-home suspension for their reckless actions on the Ashio Line. Naoto is relaxing at home when he receives a note in the mail that turns his mood upside-down: an anonymous death threat. He calls Aoi for advice, but there's no answer.