Apparently there’s a LOT of steel in that upper section to hold the structure up. The side walls are also randomly ribbed, so there’s not really a good place to clearly mark graffiti either. All up, not a bad design.

The extra width at face height makes it feel larger and more airy while passing through.

And the sloping walls help discourage graffiti by making the vandals stretch to reach the walls with their spray can (they mostly have short arms!)

There’s one of these open on Winters Road already, and probably elsewhere too.

There are different contextual meanings apart from metro systems (NY has the subway; London the underground; Paris the Metro). I saw this in Birmingham, a city that was designed for the motorcar in the 1960s. Here, subway is a term for pedestrian underground passages (usually awfully scary places; that’s why they got rid of most in the early 2000s) and underpass for vehicle-only passages as opposed to flyovers (again for cars only) that are sometimes simply called bridges in NZ or overpasses).