Is bringing back these routes a good idea?
In short - no - at least not from a business perspective.
Seen locally though, from small or rural communities with no other transportation alternatives than driving for hours or using a local airport with often infrequent and expensive service, the efforts to bring back the train is understandable, but honestly in the big picture still using a lot of money on just a few people, when large populations in other parts of the country are just as underserved.
Generally speaking trains make most sense in medium length corridors. Services under 3-4 hours will usually kill the air market and be an attractive alternative to driving, especially if the speed is not too uncompetitive.
Runs up to a bit more than double of that will attract significant ridership not least if there is several midsized stops on the way, which might not have close by larger airport. Overnight runs between larger cities are also attractive.
All Amtraks LD routes (except the auto train) are losing between 20 and 50 cents per passenger mile (YTD, March 2011 monthly performance report). One train a day runs over distances taking multiple days are expensive to run and virtually impossible to get a good economy in. Running long stretches through very lightly populated areas with no significant local one-town-to-the-next-markets to fill up seats exacerbates this problem. And on a third of the way any market is more or less killed because they have to be served at odd hours.
Instead the money and the focus should be on getting faster and more frequent service in alredy served corridors, and getting service into unserved corridors with large potential. That will get many more riders and contribute much more to solve the nation's transportation problems.
The most underserved areas today, when you factor in population size and density is in my oppinion the Southeast and Texas. The potential in the Texas triangle is obvious. I also think Atlanta screams to be developed as a hub for services to Memphis, Nashville, Macon, Charleston, Savannah and north Florida, as well as decent service on the route of the Crescent. The LA-Las Vegas-Phoenix triangle is also worth looking into, and so is quite a few in the Midwest.
Yes I know that states like Georgia and Texas have done nothing to advance train service, but that is another discussion and doesn't make the potential for trains there any smaller. On the other hand states like Illinois, Michigan, Missouri, Washington, Virginia and North Carolina are developing excactly that sort of service, often with considerable succes in spite of the fact that the current round of speed upgrades haven't started kicking in yet.