Believe it or not this is a train station entrance! The one of La Gare des Invalides, which goes to the suburbs - that's the line that takes you to Versailles for example. I never knew there was an entrance there - and to be honest I'm not even sure they really use it even though ther is a stair case and track signs. Another question is... what happens in case of floods? For this entrance is right by the Seine.