I am not personally familiar with this line and certainly the segment requiring the most work, but those that are, what are they likely doing? This almost sounds a complete rebuild. How many miles are we talking about? From descriptions the one segment seemed to be in pretty rough shape. Hopefully this extended shut down will be worth the trouble to all concerned in the long run.
The section needing the most work is a lightly used CN freight line. Someone else here knew the frequency of use and posted it, which I don't fully recall, but think it was maybe twice a week? Most CN cross border traffic in the general area goes to the New England Central, former GT, through St. Albans (former Montrealer route), not much to CP's former D&H at Rouses Point.
It was always slow, 25 mph, but during COVID apparently CN let maintenance slip on part of it to the level of a glorified industrial lead. The really, really bad part is only 5 miles, to Cantic, where the line to St. Albans splits off. But I kept hearing 25 miles of 10 mph heat related speed restrictions, too. I imagine the deal struck with CN might involve up to the whole 42 miles of the Rouses Point Sub. When railroads get money from someone else, they tend to try to get as much as they logically can.
The whole line is mostly stick rail. I imagine they may relay it with second hand CWR, although stick rail is inherently more forgiving of temperature extremes. Maybe they discovered subgrade issues with the railbed that must be addressed, causing the time extension. No one really knows, Amtrak, CN and the actual source of the funds (NYS DOT?) are being very closed-mouth about the whole thing.
Finally, the stated objective is not to raise the speed on the line, but rather to ensure the ability to more consistently operate at 25 mph rather than being subject to miles long 10 mph speed restrictions when the weather is anticipated to exceed 30°C/86°F.