The implication is that those 2 elementary students were going to a hotel right? Between them and the ones on the train trying to act all adult what happened in between Hikoichi and Nayu leaving elementary school and those kids joining that made them grow up so fast? They start listening to drill rap? They start watching Sex and the City?
When this is finished I want a spin off showing the manager meeting his wife at the office and the entire office starts shipping them in the same way.
It was implied previously that they get off at different stops so it's a remarkable coincidence they happened to be at the same station at the same time here, not impossible though, they could just be at a big station in the city rather than their usual commuter stops.