:::: The issue here, as I see it, is that "Stadtbahn" and "Premetro" are not synonymous terms in modern contexts - "Stadtbahn" and "Light rail" are the synonymous terms. Yes, a number of Stadtbahns did start with the idea that they'd be converted to full U-Bahns later on (which is very "Premetro-y", and many Stadtbahns essentially became what I guess were referred to as "Semi-Metros" in the 1970s). But, in modern contexts, as opposed to the original 1970s ones, "Stadtbahn" is basically the German term for "Light rail".