This heroine is a girl who has never ridden the train before (because her mother hates this train). She was invited to party by her grandfather, who never met her.
She rode her first train to go to mansion, and meets hardcore train otaku.

On the island of Okinawa, Sorami Karigaya is a high school girl who lost her father many years ago, and her mother just passed away. Two months after her mother's death, an attorney from Hokkaido arrives at her residence, claiming on behalf of her maternal grandfather with inheritance issues, who had been claimed long dead by Sorami's mother. The attorney explains it as a lie because for some reason she hated her father so much that she didn't even reveal his existence to Sorami. After arriving Hokkaido, the attorney tells Sorami an additional condition: her grandfather can only be reached by an express train named Genya, a type of transportation her mother prohibited her from taking.