Nicholas Beaudrot asks:
Are American metropolitan areas outside of the Northeast Corridor dense enough or well layed out enough to support inter-city rail? Beyond putting light- or medium-rail in those cities, how much would have to change before inter-city rail made sense as a way to travel from, say, Milwaukee to Indianapolis?
As a general matter, rail works better as an alternative to driving when the destination is someplace with a good walking/transit network. That said, for a lot of trips it's not really necessary to have a car at your destination even if your destination is a very car-dependent area. I went to Forth Worth for a conference once and both the hotel and the convention center are in Fort Worth's smallish walkable downtown. Combined with a cab ride to the Fort Worth Cultural District to see the Kimbell Museum and the Fort Worth Modern that contained plenty of things to do for a few days without the expense of renting a car and that kind of thing is reasonably common for business travel.
Second rail is not only an alternative to driving, but also an alternative to flying. There are a lot of flights between Portland and Seattle, for example or between Chicago and Detroit. Any time you have two cities that are pretty close by and serviced by a lot of flights, you have a situation where a good inter-city rail option would attract customers notwithstanding any issues related to the density of the destination city. Replacing air trips with train trips is good for the environment, and any time you have a viable rail option that'll displace some of the intercity car travel which is also good. Meanwhile, a passenger rail hub can become a focal point for neighborhood development and a node on a growing urban transit network.