So the fast service shouldn't use that alignment. There must already be population centers that are skipped by the fast service - its tracks don't have to go through there either.

Today the fast services follow the same alignment.

Rerouting so far has been a headache; in New Haven-Boston where the tracks are so curvy new tracks have to be built, the FRA proposed going through entirely different population centers (the geography of the existing coast line is very challenging). This has resulted in an interesting fight where people want the train in their town, but also do not want the impact of newly constructed tracks through their town.