Hello bejant, the live steam model railroad hobby has been around since the 1930's or 1940's in various sizes (starting with the smaller ones like 2.5" then 3.5" and 5") so in english-speaking countries the names were set long before we moved over to the SI units. I don't know what it is like in other places, but I imagine in France they would call it 127mm as they were early adopters of the whole SI thing. But that's only a guess.
So I have some more pictures of updates.
More details in the Roof, with some grills, some fans and a basic rib/stringer structure to hold it together.
There are also some lumpy blocks underneath for placeholders for the Bogies. I'm not modelling them as I don't have much in the way of dimensional details for them, and I intend to purchase cast aluminium bogie frames anyway.
Looking at the detail of the end of the roof on the body, these hinges are designed so they should alow the roof to close down in the gap in the body with no externally visible hinge parts. Also, hopefully they will not foul on the cast aluminium cab ends that go in the big gaps.
Anyway, hopefully I will know for certain in a week or two as I just sent out emails to ~6 laser cutting companies to ask for quotes on the pieces (in Ply or MDF, not steel yet) so I should be putting together something pysical in a week or two.