I couldn't think of anywhere else to post this, but it might be important for future reference:
Earlier I was exporting the components of the Trinity model to GM format, and it was all going well until I got to the yards, which wouldn't open in TOOL once exported.
This problem is a long-standing mystery, and before I've always used a workaround of sorts, but doing so for about 10 yard models would get very tedious.
So, I went and tried as many ways I could think of to get the damn things to export properly, and long story short, I might have found a working solution.
What I did was select a yard and go to 'Edit Polygons -> Split Vertex', deselect the yard, and then select it again and go to 'Edit Polygons -> Merge Vertices'.
This resulted in a
smaller number of faces, vertices and edges than the yard originally had, but it
looked exactly the same as before.
For some reason, that led to a successful export, with TOOL opening the file properly.
I reckon there must have been some unnecessary faces, vertices and edges that were somehow created in the original modelling process, but I still can't be 100% sure.
In any case, this might be a good solution to prevent bad exports in future...