In my opinion, some of the best views of the Portugese city of Porto are from the top deck of the Ponte de Dom Luis I. This double decker metal arch bridge opened in 1886.
I’m not sure that the current set-up of a pedestrian walkway alongside the Metro tracks, with no barriers would be permitted in the UK.
However it does mean that you can walk over the tracks to see the view from both sides of the Ponte de Dom Luis I.
When the weather is hot, there usually seems to be a cooling breeze along the River Douro, making this a pleasant walk.
The easiest way to access the top deck of Ponte de Dom Luis I is to take the metro to Jardim do Morro, on the southern side of the bridge.