Is High-Speed Rail Good Stimulus?

That depends on your criterion for good stimulus. 

If the only requirement is that the government write checks to people who are nominally employed, then building high-speed rail is great - but so is paying people to dig ditches and fill them up.

If the requirement is also that stimulus projects pass a standard cost-benefit test, then high-speed rail does not appear to measure up:

In the face of high energy prices and concerns about global warming, environmentalists and planners offer high-speed rail as an environmentally friendly alternative to driving and air travel. California, Florida, the Midwest, and other parts of the country are actively considering specific high-speed rail plans.

Close scrutiny of these plans reveals that they do not live up to the hype. As attractive as 110-to 220-mile-per-hour trains might sound, even the most optimistic forecasts predict they will take few cars off the road. At best, they will replace for profit private commuter airlines with heavily subsidized public rail systems that are likely to require continued subsidies far into the future.

Nor are high-speed rail lines particularly environmentally friendly. Planners have predicted that a proposed line in Florida would use more energy and emit more of some pollutants than all of the cars it would take off the road. California planners forecast that high-speed rail would reduce pollution and greenhouse gas emissions by a mere 0.7 to 1.5 percent—but only if ridership reached the high end of projected levels. Lower ridership would nullify energy savings and pollution reductions.

I have nothing against government infrastructure spending in principle.  But since interest groups like construction companies and unions have a strong incentive to oversell the benefits of these projects, while the green lobby has a religious hatred of cars, we should not be surprised if dispassionate analyses finds that the cost-benefit case for these projects is lacking.