Taking "the northeast" to be New England + New York + Pennsylvania + New Jersey, it covers ~135,000 square miles and has a population of ~56 million people.
New Mexico alone has an area of ~121,000 square miles and has only ~2 million people.
This matters. A lot.
It's harder for a train to go 200mph through densely populated areas where NIMBYs don't want anything built. Cows, on the other hand, don't care. The train also doesn't have to stop very often -- a train can stop in Albequerque and Santa Fe, and zip north to Colorado Springs and Denver.
The biggest challenge to high-speed trains in the US is not population density or technology; the problem is political.
Those rural areas are also largely divided into private property holdings, which have to be resumed meaning compensation for all the property owners.
You're right, the problem is political, in that the politics of the country is set up in such a way that a centralised body cannot just go and throw railways everywhere. But that is a design feature, not a bug. The same governmental processes that stop people ending up with a nuclear waste dump next to their town also apply to railway lines. You can't just plan this stuff from a central office in Washington and make it so.
The real problem is economic, not political.
A NY-SF coach-class train ticket cost $65 in 1870. That may not sound like much, but that was 2.8 ounces of gold -- figure $4700 at current prices.
A first-class ticket with sleeping compartment cost $136 -- almost $10,000 in current dollars.
Right. Because they don't have vehicles, chickens or wildlife in any of the other nations with high-speed rail networks. These are clearly unsolvable problems.
Sometimes it seems like people are just reaching to find any possible excuse to dismiss the idea based on ideology alone, doesn't it?
As far as the problem with wildlife and trucks and rural roads—you just need a grade-separated track with passages above and below. Expensive but necessary and plausible.
There was talk of a new rail line to the south of Rochester, MN a number of years ago and some landowners were unhappy because the proposed line would split their fields, require new fencing to protect livestock, etc.
Here in Oregon, other landowners have gotten letters from the railways demanding payment for driveways across their lines just so they can get to their property. See http://www.kval.com/news/local/Neighbors-on-edge-over-new-ra...
Would it be easier to force a half a dozen farmers/landowners to give up part of their property than thousands of house/apartment dwellers? Of course. But its still forcing people and that results in lawsuits which will drag things out.