If there was any kind of joined up thinking in this country the M1 viaduct could have had a train deck. I travel across one morning and evening - designed 1918, decades before the subway eventually ran in 1966! The M1 viaduct is 140m shorter and 20m closer to the ground so you never know what could have been accomplished.
http://en.wikipedia.org/wiki/Prince_Edward_Viaduct