Ive added a Zip file called Y_carriage_dragchain_8x10_STP_files.zip to Thingi see if they work.
If they don't work or you need other files let me know.
I thought Solid works imported STL files natively. With Inventor I have a addon that allows me to import a STL and turn it into a base feature then I can modify as a part file before exporting it back into a STL.
Very handy with working with Thingi files.