The "hn" he makes in response is entirely transparent in its false grumpiness. He's a grumbler by nature but he rarely means any of it.

Because he's doing his best not to stare at Erik, the nudge surprises him slightly, though John doesn't jerk or anything. The fondness for the train is in its own way also startling, but perhaps endearing, if John were susceptible to that kind of thing. He obligingly follows Erik to the end of the platform, and presumably, at some interval, onto the train.