The important long-distance trains ARE high-density service. Or would be if they ran on time.
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I actually agree with in you in principle, at least generally. I do think that some long distance trains work as a series of overlapping corridors, like the LSL or the Silver trains. There are probably other examples, either that could exist or perhaps parts of existing routes. I am less convinced that is true of the Western trains, although an argument could probably be made for the EB as essential transportation based on the limited Interstate highway access on its route. But anyway, the idea of building, as you say, a reliable and timely network of long distance trains is tilting at windmills when there are more important and urgent concerns with the existing network.
In order to have reliable and frequent passenger service you need to own the infrastructure. I know you have made that point before, and you're right. But how do you do that? Either you forcibly take over the existing freight trackage, or you build your own, and neither of those are realistic. If you think there is political support to seize major freight corridors, you're out of your mind, and it's a horrible idea anyhow in terms of national infrastructure. Intercity passengers can be, and by and large are, accommodated via highway and air travel, but there is no realistic way to transport the volume of freight that moves by rail if that track capacity is taken by passenger trains.
Building a parallel passenger route (actually logistically it would probably be better to build freight lines to preserve passenger access to downtown cores) is maybe
marginally more politically feasible but far more costly and would almost certainly involve decades of legal wrangling over eminent domain cases at huge expense. California's struggle with HSR is certainly not a model you would want to follow, but would probably look like a cakewalk compared to a similar project using exclusively federal funds across multiple state lines.
And in any case, the United States struggles mightily with even maintaining its existing publicly owned infrastructure, both road, rail, and in some areas aviation. Why should we be spending precious time and money in building a brand new system when we can't even get the existing house in order? Unfortunately there is not an unlimited pot of money to go around--and this is where I will complain again about the Downeaster, because both Brunswick projects I mentioned were primarily federally funded--and I think that the investments should be going to where there are identified problems with current infrastructure, such as any number of NEC projects, the NS mainline entering Chicago from the east, the painfully slow Chicago/St. Louis suburban running on the Lincoln Service, CSX chokepoints in Virginia, etc.
Perhaps you are more optimistic about the chances of the government suddenly managing to develop a bipartisan consensus to make a huge infrastructure push than I am, but I don't see that happening in any of our lifetimes. Ironically, even as public support for public transportation will probably continue increasing gradually, the political support has likely peaked as the left/right urban/rural split continues. I don't think that either party will ever control both houses of Congress and the presidency with a majority strong enough to push through effectively single-party legislation, which is really what would be needed to pass the kind of legislature required to build out some kind of national rail system that is significantly different than what we have now. So most, if not all, major public transportation projects are going to come under the auspices of state or municipal governments, which pretty well precludes the idea of major interstate developments.
I guess what it comes down to is that at this point I think the balance of both the nation's existing transport network, and associated travel patterns, and the political support for public infrastructure investments has tipped so far towards the use of highways and aviation for long distance travel that it's a futile endeavor to advocate for a wholesale restoration of the national passenger rail network that existed up until the 1950s. Expecting that to come back is going to be a long wait for a ship that won't come in. Instead we should be directing what money is available towards projects that will support the continued operation of trains in the areas where they are already an important segment of the transportation network. Expansion of the network is going to be led primarily on a regional basis, such as what we have seen in Virginia, some areas of the Midwest, and California. If Indiana or Ohio or wherever doesn't choose to make the effort, well, that's a pity, but there's no point in spending political and monetary capital in trying to force trains upon them when there is so little of it to go around and so many other places to spend it.