That couldn't be just a coincidence, I had to found that railway line!!! And yes, after some research I found this line is called Chemin de fer de Petite Ceinture (French for "little belt railway").
"...was a Parisian railway that, from 1852, was a circular connection between Paris's main railway stations within the fortified walls of the city. In a partial state of abandonment since 1934, the future of its yet-undeveloped property is the subject of much debate as of 2013. Many French railway enthusiasts and associations would like to see the abandoned railway and its remaining stations preserved and protected as a part of France's national heritage."
I also found there is a park called Montsouris in the south of Paris, crossed by the normal railway line and also with a visible part of the Petite Ceinture. Now I can assure the photo was taken exactly in this park, it's exactly the section of tracks passing the park Montsouris.
And these are my own photos. As my camera was broken in a previous trip to Germany, I just had the mobile phone so the quality is really poor:
The line extends outside the park, may be in another trip I try to follow it and see if there is a point to go inside.