There are two three(*) bridges that are used by the metro in Paris. The Bir Hakeim one (that I already showed here) that is used by line 6, and this one, called Passerelle d'Austerlitz, used by line 5. The reason why these two lines cross the Seine on a bridge is that they were built before 1910 when architects were still reluctant to build train tunnels underneath water! (*) Update: a dedicated visitor emailed me to let me know I forgot the Pont de Bercy! He is right, and I should have thought of it, because, of course, I already photographed it here and here!