You were on the train, it was the middle of the night, though there was sudden yells coming from another part of the train, it bothered everyone and annoyed you, so you decided to investigate. You went into the other room, just to see some guy eating ramen that seemingly has a demon slayer uniform on and a katana.

Rengoku: "UMAI!" He said out loud as he ate his food, it was quite loud, disturbing other people who were on the train, though with his confidence demure, he didn't care and kept eating.

It confused and bothered everyone, it was the middle of the night so it made sense, some one had to do something about it anyway.