There are tons of city pairs but because the airline industry is heavily subsidized it's often cheaper to fly.
The Boston to Washington D.C. corridor could easily sustain itself if the trains put in place were reliable, expedient, and low-fuss.
There's no technological barriers here, as China has proven, only political ones. A lot of people see Amtrak as a problem, an anachronism, not a solution.
Airports are a spectacularly expensive undertaking and require heavy subsidies to even exist. If the airlines had to pay for those with cash they'd be bankrupt unless they charged more. Then there's the subsidies paid to the oil companies that makes jet fuel more cost effective, and airlines burn a lot of that. As a portion of net cost, fuel is likely higher for aviation than for any other form of travel.
$250B sounds like a lot of money, but that's since the 1970s. In that time how many airports have been built or rebuilt? What's the net cost of that?
The footprint of a rail station is significantly smaller, the costs are proportionately lower. There's subsidies there, too, but they're significantly smaller, and many of those were provided in the early 20th century back when rail was a vital link.
Also Forbes is utterly useless as a site. Not only do they bitch endlessly about ad blockers, which I only use because the sorts of advertising sites like that promote are offensively stupid, but it splits up that short article into three pages with interstitial ads between each page.
The corridor does sustain itself. In fact, it basically makes the money that Amtrak loses throughout the rest of the country. It doesn't really work (for most people) for Boston to DC trips but it's quite popular for NYC north and NYC south.
The Boston to Washington D.C. corridor could easily sustain itself if the trains put in place were reliable, expedient, and low-fuss.
There's no technological barriers here, as China has proven, only political ones. A lot of people see Amtrak as a problem, an anachronism, not a solution.