3D scanning is the usual process used nowadays for the majority of model railway locos. As my post below one UK minor manufacturer showed examples of the "raw" 3D scans (taken at various positions, including from high level of the roof) the result was an amazing mess with lots of spurious lines, fuzziness and mismatched features where adjacent scans didn't quite match up. Recently two minor manufacturers owned up to "minor" errors, one a deliberate to make it possible for the wheels go round (!) the other something missed due to being masked from the scans.
The huge mass of scan info had to be cleaned up, matched and "tweaked" by someone who knew what he was doing and who had a mass of reference photos.
THEN, he had to adjust them into files suitable
to be turned into 3D CAD files WITH adjustments made to "turn back the clock" from the preserved locomotive now with modern changes back to the condition it ran in in the desired time period.
LOTS of work.