If you are using a train in the UK, then a Brompton is classed as luggage. (This is the main reason I bought mine - it means you don't have to book a place on a train for a usual type bike.) Given the current Covid situation, I've yet to experience any complaints when I pop mine beside me where I sit, because of the lack of people, but am prepared to lug it onto the luggage rack if, and when, it becomes necessary.