It's good that you are attempting to argue this based on cost, but you do realize you have ignored federal subsidies for the automotive sector, correct?
Ignoring the costs of war (just to keep the argument simpler), the feds spend less than 1% of transportation spending on rail and the rest goes to build new highways (most of which doesn't serve to help traffic, but does serve to build access to land owned by well-heeled developers).
Hidden subsidies make this argument impossible. 90% of Americans believe the federal has tax accounts for a substantial amount of the price of gas, AND that the gas tax pays for the roads. I even see letters to the local paper asking why the police don't arrest bike riders for "stealing" space on the road without paying taxes. The majority of the cost of road construction comes from 2 places: income taxes, and loans from oil rich states (who don't want to give us a reason to quit).
The places you mention do make the most sense for high speed rail, as well as pockets in Florida, urban rust belt, and parts of the west coast. DC isn't a "maybe"... if it were safe to bring a bike on the highways, you could beat traffic home whether your commute were 5 miles or 15... DC is a congested mess.
This isn't about "replacing" any method - that angle smacks of taking away choice. This is about expanding choices.
We have 2 wars going on, and we're not making any economic sacrifices for them so the day of rekoning will be that much harder. My grandmother told me stories how during WW2 you would be berated for NOT carpooling, not growing a 'victory garden', and not recycling. We were promised we could ignore looming economic problems and it would sort itself out.
For years, people who talked about "peak oil" were dismissed as making it all up for some socialist agenda. If you want to be really scared, take a look at today's headlines about Saudi Arabia misrepresenting it's oil reserves by 40%, the US government knew it but kept quiet. Thanks to Wikileaks, we know the truth now. (Although it was really kind of obvious once the rest of the word got developed, that oil couldn't remain the domain of any 1 economy)
Now we're about to EXIT a major recession - a recession which has tempered worldwide oil consumption. Other economies are bidding for the same barrels of oil, and will be able to outbid the USA because we don't MAKE anything after outsourcing and offshoring it all away. That's like playing a game of poker where you keep bluffing -and- borrowing from the other players at the table.
High speed rail won't grow our economy - nothing can do that except a return to manufacturing. But without high speed rail, we're going to lose a lot more dollars to the middle east, or maybe we'll rely on Russia for energy...
I didn't even bring up climate change, because frankly it is so politicized :-(
I'm not pro-coal, but it is worth pointing out that even if some high speed electric rail were powered by dirty coal, those trains would have far lower CO2 emissions than the same number of drivers in cars. In a lot of places, cars idle for much of the time, and under the best conditions gas vehicles achieve less than 15% efficiency which is far less than a coal plant (plus a regional power plant can be upgraded to run cleaner FAR easier than all the cars in that area getting individual pollution upgrades, which is not feasable. It is interesting that energy profits are privatized, but air/land/water pollution is socialized.
Oh, and airlines? They're only cheap because the taxpayer supports them.
During good years airlines do not save money for rainy day - it goes out as dividends and bonuses. Instead of investing in efficiency, airlines lobby for taxpayer-underwritten loans... which a few years later (rainy day) the airlines default on. That defaulted debt is passed on to the federal government.