Comment by SoftTalker

The NYC subway (and elevated) trains were also originally operated by several different private companies, and there are still at least two major types of rolling stock (used separately on "A" division and "B" division lines). Maybe less variation than London, but I really couldn't say having never been to London.

Chicago's "El" and subway system AFAIK, uses the same rolling stock on all lines though there may be a couple of different generations of cars in service.