The abandoned-railway book by Paul Berna: I read it --
@AndrewE, like yourself about 60 years ago (it was in the library at my preparatory school), translated into English, with the title of
Line of Attack. I've forgotten the names of any of the characters; but I recall that it was set in Brittany (seemingly a favourite setting of the author's, though I gather he himself came from the South of France) -- the characters were the children of two traditional-mutual-enemy villages called Belmont and Camarec; the kids from one, managed to get working a diesel railmotor of the closed line, to use in mock warfare against the other crowd.
Googling attempt re Berna led me to mentions of various of his books; none there however, of the "railroading kids" one as above.
@citycat:
The Horse Without a Head (
Le Cheval Sans Tete) has been -- per my Wiki and elsewhere findings -- translated under the title of
The Hundred Million Francs -- and, again per those recent findings, is about a bunch of fairly rough, tough Parisian kids (Gabby & Co., as mentioned by you?) -- one would figure that the Bretons with their railway, would be different characters. I have a feeling that I
might have read
The Horse / The 100,000,000... -- later than at school circa 1960 -- but have absolutely no memory of my own, of the content.
Had never, before this thread, heard of Etienne Cattin and his
White Rat books -- feel inclined to seek them out !