Located in ./GitHub/RailCOMPLETE-DNA-[XX-YY]/[XX-YY]/3D.
RailCOMPLETE expects a 3D model library. This should be a collection of folders, each containing one dwg file for each of its 3D models. The path to each such folder is set by the DNA through the <Geometry3DLibraryFolder> element.
See here for a general guide on how to make 3D models in AutoCAD.
For guidelines on how to make well designed 3D models for RailCOMPLETE, see the 2021-01-08 002 Instruction manual – 3D modelling with RailCOMPLETE.pdf file on the RailCOMPLETE download pages under the 3D Modelling Instructions tab.