Passenger:
noun
a person who is traveling in an automobile, bus, train, airplane, or other conveyance, one who is not the driver, pilot, or the like.
Customer:
noun
a person who purchases goods or services from another; buyer; patron
A railway passenger/customer fits both definitions.
TBH I don't overly mind which they use if they sort out the service. What I don't like, though, is
excessive informality. I'm not the guard (or whoever)'s mate, I'm their customer.
But I think I may be a generation behind on this. I get
particularly annoyed with sales calls which call me by my first name, but I bet younger people don't.
And I also find a bit of olde-worlde charm in some railway-isms (though LNR's "this train has N carriages" is a maybe a *bit* much), so as long as they're not confusing...