Hi,
here are free plans for a 45mm Decauville. There are some DXF for CNC machining. Two or three were built.
http://jpduval.free.fr/Lv020de45%20plans.htm
http://jpduval.free.fr/Plans/LV%20020%20DE%2045%2003%2010%20B.pdf
http://jpduval.free.fr/Galeries_photos/Decau-en-vrac/index.html
Patrick