Should Liverpool's Merseyrail Count As The UK's Fourth Underground Railway System?

Cities that can't afford an underground metro system build a tram system but act as though it was a metro (looking at you, Manchester). Cities that can't afford a tram system build guided buses, but act as though they were trams (looking at you, Bradford). So why do (some, at least) Liverpudlians insist that the Liverpool Merseyrail isn't an underground railway? Because to this Londoner, is looks like this one. My daughter and I took a look...