The train that goes from St Valery sur Somme to le Crotoy in one direction and Cayeux Sur Mer in the other (Chemin de Fer de la Baie de Somme), is a steam hauled service dating back to 1887 and does not seem to have changed much in over a hundred years, and is well worth a trip on.
It takes you through woods and hills, along the banks of the Somme and stops in several towns such as Saint-Valéry.
No trains on Mondays!
And as we were there on a Monday we missed the view of the steam engine. Too bad.
The picture shows the deserted station in "Le Crotoy", the town at the other side of the estuary, where the train departs.
The website down below is in French. Couldn't find one in English.
The Baie de Somme is located in the North of France, on the Picardy coast approximately 2 hours from Paris and 1 hour from Calais.
It's possible to travel around the baie de Somme by this old steamtrain from April until October. You even can take your bicycle along. You can start out in Cayeux, St. Valéry sur Somme, Noyelles sur Mer and Le Crotoy.