(from ebookjapan):

There is a gray wagon that appears with an eerie, whistling sound that seems to come from the depths of hell... The history of the gray wagon told in old tales has been passed down to the present day, and once a week on Fridays, it would hunt and collect juvenile delinquents.

The delinquents gathered in this way were taken to the "Sea Cage," a juvenile delinquent ship that held 300 delinquents, one-third of whom were tortured and physically punished before their lives were plunged to the bottom of the sea. It was a reformatory on the sea!