So I have kind of a weird question. On the Hogwarts Express, the students change into their robes partway through the trip. All canon evidence (save for the illustrations) points to the idea that wizards' robes are like dresses - you don't generally wear anything under them. So how do you think they'd change: in the compartment? In a bathroom? If in the compartment, that implies that the doors on those are solid and opaque - hopefully. Does anyone know if there's any canon evidence to the contrary?

Thanks ahead of time for the help.