This is only a short stretch of a very long walkway linking two lines at Nuñez de Balboa metro station, Madrid. (I waited two whole days to get a shot without people!)

There must be a reason why all these long interchange walkways are all on a dog leg basis rather than more direct in a straight line. Something to do with following the road grid network; perhaps this was the only way to build a new line in the 40s and 50s. The 80s and 90s expansion of the network used powerful deep tunnel borers so following a road up above was no longer necessary. (Pure conjecture on my part!)