Trimming millions from the cost of the Dulles rail project could prove a risky operation for Virginia and Metropolitan Washington Airports Authority officials, with any change that cuts ridership likely to prompt a stern re-evaluation from the Federal Transit Administration.

The FTA on Monday released a report challenging the official $2.65 billion project cost estimate for the first 11.6-mile phase of the Metrorail extension, saying the track would be substantially more expensive.