Maybe they partially planned for it? Could they re-grade later? If there's plenty of diggable stuff underneath the tracks (just dirt and ballast, no roads/metros/pipes underneaths), they could easily regrade the tracks later. Tracks can be regraded gradually inches at a time overnight without shutting down service or for a weekend -- it's been done before. Huge trackside machine lifts track, digs or adds ballast, lower track. This is also done for lateral track shifting, e.g. shifting the tracks sideways small amounts at a time, to make room for new trackage or make tracks straighter. Several weekends later, with no weekday service shutdowns, you've upgraded your speed rating from a series of gentle successive weekend regradings and trackshifts. I imagine that a process can be invented to nurse the grade curvature to 100mph later.
Some showstoppers:
- Mix of being overpasses & being underpasses, make it complicated to re-grade. There are many sections of Georgetown corridor that goes over roads, so you can't sink the tracks easily.
- More trenchwall may need to be added, as they make the grade curvature gentler for faster high speed trains. Did they make the existing trenchwall deep enough for future modifications; etc.
- Complications underneath (e.g. pipes, rock, etc)
HSR regrading wouldn't be something they need for about 15 years (10 years my arse, it's gonna be at least 15). So it may not be something they needed right away. Get something built quickly, but design/plan it in a way for future regrading. Maybe overpasses/underpasses down the rail restricted the grade curvature. THAT would be the more expensive part, and limits the re-grading options, and may explain the 75ph limit. Otherwise, to meet HSR grading criteria, you may end up having to convert down-track overpasses over roads into underpasses underneath roads. This process can become incredibly complicated.