Thanks so much! This helps a lot.
The non-watertight sections are mesh artifacts. Guess I should invest the effort in building the mesh better 
So Hexahedra tend to be more efficient than Polyhedra (at things like stream lines at least). To build a watertight mesh, I’ll probably have to explicitly make a mesh of Tetrahedra. Do you have any insight whether this would be more or less efficient than something like ConvexPointSet? This probably suffers similar triangulation issues to Polyhedra, but would be less objects than Tetrahedra…
Alternately, is there a filter that adjusts a mesh to fix small overlaps/voids? My first implementation of the reader used a MultiBlock dataset of StructuredGrids, one per track. This was much faster, but since the tracks didn’t know about each other the overlaps/voids were too significant, could easily be seen by eye. A filter that would take in this and build a watertight mesh would be perfect, but this is probably too niche a task for a general filter.
Thanks again!