High-speed rail requires a massive up-front investment that air travel never did. If it works (in terms of ridership) as hoped and enough people ride it, then those costs can be recovered. But if it doesn't, or if too few people decide to take rail, then its tens of billions of dollars down the drain. And success, while it may be possible, sure as hell isn't guaranteed. In particular, cost estimates of construction could be far too low, especially in places like California (which has to worry about earth quakes, mudslides, and environmentalists).