IIRC there is such a thing as a clearance diagram which locomotives and rolling stock will be required to meet. I don't know how these are managed. The diagrams can include effects of track curvature in tunnels, etc.
From the article it sounds like the fixed platforms themselves were built out of spec.
basically the tunnel didn't allow enough clearance for a double track in certain points, so a single track was placed (temporarily, for Itala 90 world championship, some 30 years ago, the tunnel was used - with single track - for a few days and then closed, it will probably be reopened in 2022).
IIRC what happened in France was indeed that the platforms were out of spec, or perhaps the specs were out of platform. The rolling-stock people asked the tracks-and-platforms people for the specs, and got the latest version. The fact that several older platforms still in use were built to an earlier set of specs got lost somewhere along the way...
They're managed with much effort, at least where I live. There are so many things near the track (in a sense that's why the track is built in the first place). Florida man may have skimped on the effort.
