My first choice is city center to city center between cities with high business travel by car or air, where the train travel would actually be faster and presumably cheaper than either automobile or air. Some examples include:
- NE Corridor: between Washington DC, Philadelphia, New York, Boston
- Texas: connecting DFW, Austin, Houston, and San Antonio
- North Carolina: Charlotte, Triad, Triangle
- California: San Diego, LA, San Jose, San Francisco
I am sure there are others not mentioned here. The most obvious routes in the above list are the NE Corridor, and the Texas Routes. I have concern that it will be hard to beat air travel between LA and the Bay Area destinations, but considering airport security wait time, it still may be viable.
The economics of the high speed rail will most likely return a profit only if it is able to replace air travel, and some auto travel between destinations. It the business case is profitable, it should attract private investment. If it is not, it doesn't make sense for the government to spend on it either. As much as I want high speed rail, the $53B investment may just be too much, too soon.