Where I grew up, cities either had trams (rarely) or subways (slightly less rarely). London started the subway trend. (As a note, the largest metro/subway system is in Shanghai. London comes in tenth, and New York sixth).

Elevated trains were something I heard of in precisely one context: TV/movies set in Chicago. The L is a symbol of the city, easily visible and recognizable. But Chicago is not the only place to have elevated trains (And as a note, Chicago also has subways…some of the newer lines go underground instead).

Both subways and Ls solve the same problem: A tight space and limited rights of way.

So, who first thought of putting the train above the road, and why might cities choose it over subways?