This paper examines the full costs, defined as the sum of private and social costs, of a high-speed rail system proposed for a corridor connecting Los Angeles and San Francisco in California. The full costs include infrastructure, fleet capital and operating expenses, the time users spend on the system, and the social costs of externalities, such as noise, pollution, and accidents. Comparing these full costs to those of other competing modes contributes to the evaluation of the feasibility of high-speed rail in the corridor. The paper concludes that high-speed rail is significantly more costly than expanding existing air service, and marginally more expensive than auto travel. This suggests that high-speed rail is better positioned to serve shorter distance markets where it competes with auto travel than longer distance markets where it substitutes for air.