Track geometry and maintenance
If an upgrade for higher speed is considered in Denmark, the infrastructure company, Banestyrelsen, will simply export a file out of its company database. Such a spreadsheet table allows instant calculation of many "if"s, and gives the following data:
In the days of computerized track maintenance machinery, having and maintaining such a database is the cheapest approach at the task, if a constant quality level of the tracks is the desired result.
When doing the same kind of work in the USA, the above data might not be available, or at least not all of it. It is not guaranteed, that a railway company has a chart of its track at all. If it has one, important data like track superelevation might be missing. If the data is all there, there is no guarantee, that this data describes the real-world track. And if the data is realistic, there is no guarantee, that the track geometry won't have changed after the next major maintenance work. After many decades of track maintenance, it might also happen, that a curve hasn't one radius, but three or four slightly different radii.
Possible negative results of such procedures are compensated by setting very conservative speed limits, that are often considerably lower than what is achievable even by very long and heavy freight trains.