Highways are overburdened and falling apart. Bridges have to be replaced to meet the needs of today's heavier cars and trucks. (Hybrid cars with 2+ tons of batteries, esp.) Highway widening destroys homes and businesses.
Airports have been constricted by airport-driven commercial development. Building new airports farther from downtowns is expensive and impractical, especially considering many modern international airports are 20+ miles away from downtowns.
A seven-car Amtrak train can carry 422 passengers, or 4/5 of a Boeing 777 in a Ryanair all-economy configuration, in comfort with a cafe car and business class.
A seven-car Amtrak train does not have to share facilities with international flights, nor does it have to wait for limited landing slots, and therefore can be run fifteen or more times a day.
A seven-car Amtrak train does not overburden existing infrastructure: how light is an Amfleet compared with a 70-foot heavyweight car of days past?
Highway widening, especially in urban areas, can be extremely costly. We're talking billions to widen 10-20 miles of highway near cities. (and still billions to widen highways in rural areas, since the distances are longer) Maintenance costs are through the roof: we have to repave at minimum once every 5 years for an asphalt highway. (Concrete will last 50 or more years, but that's more expensive up-front and therefore is a waste of taxpayer money, am I right guys?)
Meanwhile, Amtrak is running Northeast Corridor trains at 160mph on 150 lb rail laid by the Pennsylvania Railroad in the thirties.
It costs $10 million per year to run a train from Lynchburg to Washington, twice a day, for a year. This is a distance of 152 miles.
Imagine, for a moment, that you run six trains a day, for a total one-way capacity of 2532 passengers/day. That's 2532 cars off the road each day; that's billions saved on widening 152 miles of highway. Even assuming running six trains a day costs literally six times more ($60 million), and the highway widening costs only $2 billion for 152 miles (unlikely, given the mountains being there), that's about 100 years to break-even, discounting maintenance.
We're in a fiscal crisis. Let's take the cheaper option, and invest in interstate passenger rail now.