Rail Wars!

Synopsis

Takayama's friend, Mari Sasshō, works in a restaurant near the defense headquarters. While eating lunch, she serves them and is introduced to everyone. She will also be attending the same Academy. The next day, Sasshō's friend Kaori goes missing and she requests the Defense Four's help finding her. They go through reports to no success and Sasshō ends up hearing Kaori's phone ringtone in the lost and found. She listens to an audio recording on the phone and hears the sound of waves and specifics to a certain station with her excellent hearing. They search through warehouses along the line and find her tied up as the witness of a crime scene. They take down the criminals and take Kaori to the hospital.

(Source: Wikipedia)