In reply to MartynH:
Not on the train stops, but if you have transport, there is a nice way marked circuit incorporating the Gorges aux Chats in the north of Trois Pignons - it's marked blue on the IGN 1:25,000.
The GR 11 is marked in red and by studying the IGN you can device numerous circuits incorporating some of the rock outcrops this passes
A word of warning though. The trails are constantly changing so navigation in the Forest can be challenging to say the least!!
Enjoy!