The Loop L and the parts on structure are not much of a limitation, but a lot of the Metra Electric and the UP-North viaducts are pretty low, as is the North Side L north of Wilson. However, I think CTA will not show much interest in vehicles they have to route so carefully, that can't be interlined or even pull in to the garage except on very specific routes. Chicago has more than a thousand overpasses with clearance of less than 14 feet.
Chicago Motor Coach, of course, ran double-deckers along Lake Shore Drive and west on Addison.
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