The relationship between fares and return is difficult. How soon do you need the investment to pay off, and by what degree? The TGV recovered the initial construction costs in 15 years, but what company will wait even 5 years for a return these days?... Privatized High Speed Rail is out of the question because it is not possible to raise the capital to build and operate such a system in the private sector. In addition the fares that would have to be charged to cover operating costs and earn a return on such an investment would not produce enough ridership to make the venture worthwhile...
Also, it would be vital to use the stations as revenue generating sources. The Dalas DART system was calculated to have recovered the cost of construction in increased economic activity within 5-10 years, but that went directly to the city, so the system relies on "subsidies" of the money it generated to keep operating. The train would need its own clear revenue stream.
The most practical solution might be a public-private partnership, where the government helps fund construction, then takes a cut of the profits.
