Many thanks for the file and info. But I'm struggling to reconcile all that with what I'm already doing. I can see the model in TurboCAD but I can't make much sense of the actual file contents.
What's puzzling me is why you have generated the screw-top in such fine detail:
compared with mine:
Bearing in mind that even in 7mm scale you need a magnifying glass or a close-up lens to see the chair screw details, such fine detail seems completely wasted. It means your file is 1.3MB and a full chair is going to be something like 2.5MB. The STL format is a flat format, it doesn't as far as I know support blocks, so at 2.5MB per chair the file size for a full print is going to be massive. Templot's STL files are already 10 times the size of the corresponding DXF file (which uses blocks).
That's all a bit academic because at present I haven't the faintest idea how to convert your file to an STL in Templot. I could presumably integrate chunks of your DXF into Templot's DXF without understanding it, but I can't do that for the STL. I was hoping your file would be a much simpler DXF (such as can be imported into the background shapes for example).
At present I can see a road forward from where I am now to where I need to be -- plug-in chairs for every common size of switch and crossing, adjustable for rail section and tweakable for gauge, and with corresponding sockets in the timbers. Now that I have implemented the direct STL output, there is no actual technical problem, it is just the long and hard grind of doing it. The switch chairs are fairly straightforward, but for the crossings we have to be able to chair, say, a 1:7.38 V-crossing or 1:4.68 K-crossing. All doable, but I may be gone for some time.