As a reference, there are only 7 Acela trains per day (or at least on Monday 4/25) from Boston to New York, two cities with substantially higher population and apparent demand (as evidenced by the 59 non-stop flights from BOS to any of the NYC-3 airports on Monday 4/25)
We manage to do that between a town of 20,000 and a city of 1 million for comparison. Or if you feel commuter routes are different enough to not count, a city of 60,000 and a city of 1 million with similar travel time as google maps quotes me for Boston to New York.
In my experience over the last few weeks there's barely been an empty seat.
I took the London-Paris train last week, absolutely rammed, there's 13x 900 seat trains a day at the moment, and that has all the nonsense of eurostar (airport style security, passport checks etc).
And this is on the Northeast Acela, the crown jewel of the Amtrak network and between two cities with generally functioning public transit once you arrive. Most US city-pairs would be worse.
The Northeast Corridor service is very popular. In fact, I believe Amtrak has plans to expand it given that it's pretty much the only place in the country Amtrak doesn't lose money.
A train every hour is convenient for some purposes, but for others you just want a bunch of trains early in the morning and later in the evening.
I don't think its popularity is a reasonable predictor of demand for a modern train that would be 3-4 times faster.
I'd wager if all the other competitors were also running with tech typical of the 1920's, the train would be more popular.
Fun fact: technically the boring large Pacific Surfliner trains could be "high speed rail" since they could get to 120 MPH through Camp Pendleton if the line had PTC and was signaled correctly. As it is hits 80-90 through there, but it soon has to slow down for a stop.
To do high-speed rail right you basically need four tracks - a slower local service that stops at every stop, and a faster high-speed express service that only stops rarely.
