Mamotte Agechau!

Main character returns from a part-time job by train when he sees a drunk bounded woman alongside a shady man. Protagonist steps up for the woman, and after a fight shady man jumps out of the wagon when train stops. Girl turns out to be his former sempai at school. She invites boy to a meal in gratitude for help. During the dinner old feelings bloom, and and absolutely drunk life begins in a nearby hotel.

[From vndbreview]