I've read this as well, and have tried searching for it now without success. Wasn't there a scene outside the building where he works, where he meets the guy for the second time (ie, after the train meeting), and that's when he finds out that the guy on the train is a part of the family he's supposed to join, or something?
Is it this? If it is, then it's just a single chapter/one-shot in a collection.
http://www.mangago.me/read-manga/dame_na_yatsu_hodo_kawaii/ (chapter 3)
Direct link to the chapter: http://www.mangago.me/read-manga/dame_na_yatsu_hodo_kawaii/nbt/517d9121eaf40c44/Ch3/
Manga about a guy who ran away because he was going to be in an arranged marriage and he met someone on the train. I forgot what happened, but the guy he met while on the run was related to the family he was supposed to marry.