Some years ago I read the memoires of a Dutch newsreader on his experiences in Paris. In his book, called Gare du nord, the model trains in the basement of gare de l'est were mentioned. Here pensioned train drivers meet to discuss their past, while playing with model trains. I went there twice and must say it is quite impressive and surprising what I saw. Ever since I have suggested it to other friends visiting Paris.
However, I could only find the entrance because of the detailed description given in that book. There isn't any sign of its existence. I was never sure if this basement is open to the public, but nobody stopped me when I wandered around. I had some nice conversations with one of the participants, although most keep to themselves.
Was I an intruder? Or is this area open to the public?