Well, the only facilities with that many tracks are workshops, of which only a few are underground and physically separated from the main line and thus very unl

Well, the only facilities with that many tracks are workshops, of which only a few are underground and physically separated from the main line and thus very unlikely to accept passenger traffic as they are all dead ends. From the looks of it, you arrived at Gare du Nord and departed from Gare d'Austerlitz. The best option would have been to take line 4 at Gare du Nord and switch to line 10 at Odéon until the terminus at Gare d'Austerlitz. There is nothing interesting on the first leg beside the staircase at Cité. The second leg gets more interesting at Cluny - La Sorbonne, where the two tracks become three, and right after Maubert - Mutualité, where the two tracks split to make way for a third track to go under them. That part is mentioned in the parent article as "two-level crotch". There are other interesting places on the same line, like the "triangle" between Porte d'Auteuil, Michel-Ange Molitor, and the Auteuil workshop but they are in the other direction. Another thing to note is that the tunnels of line 10 are painted in white, which makes it look cleaner than other lines.