
I was merely suggesting a name for adding variety to your set, if you want for it being more "universal". but that's up to you, of course.
I'm going to try to find infos on Decauville, unfortunately, most of the pages I found are in French, obviously.
Plus many of them have lot of info about the use of the railways, but little in the technical side... If you have the power output and the size of the locomotive, you're already in luck :/
You might have more luck looking on the "Système Péchot-Bourdon" based on Decauville for the French army, with the most prominent feature being the locomotive, a Janus-type steamer :

I'm not sure that OpenTTD will allow a double smoke, as it would be what the loco would do (there is one common boiler on it, so both chimney emitted smoke... apparently, making it an easy target during war

http://www.wikiwand.com/en/Trench_railw ... _equipment
http://fortiffsere.fr/forts/index_fichiers/Page1549.htm some data here, in French...
Incidentally, I found this : http://www.cftt.org/pages/uk/day-steam.html
Now I have to go there and try that!
And I see you got links when I was searchign and typing. Good job Michael!