The story is told through a series of notes left by characters the user finds in the train.
How the stories are made
The train has as series of pre-defined characters
Each character has one or more stories
Each story has a fixed structure and theme
Each structure is made up of a fixed number of letters, notes left by the character
Each letter contributes to the story in line with the theme.
However, each letter may have multiple different versions that each tell take the story in a different direction, but all still fit together.