"I told him I had this relationship with Kimon Proussaloglou and may have mentioned Cambridge Systematics in a throw-away line," Koppelman said. "He asked me if it would color my judgment. I assured him it would not." Van Ark has been unavailable for comment since he left the agency last month.
Koppelman said the rail authority did not ask him to file a statement disclosing income and economic interests, which are usually required of government officials and contractors in decision-making positions.
Cambridge officials and Proussaloglou declined to comment. The firm boasts that dozens of state, federal and international agencies depend on its planning and forecasting models, but it has remained silent about attacks on its ridership figures, except for a sharp retort to the Berkeley report.
Over the years, the bullet train's passenger estimates have fluctuated from a high of more than 100 million passengers annually for an 800-mile system between San Diego and Sacramento to about 30 million for a 500-mile network linking Anaheim and San Francisco.
In a series of agency reports over the years, Cambridge's ridership estimates have dropped sharply. The 2008 plan estimated an initial 500-mile system would carry 55 million passengers annually by 2030. In 2009, the number was adjusted to 41 million by 2035. In November, a draft plan put the number in a range as low as 29.6 million by 2040. And the business plan issued last week dropped the middle of the range to 26.4 million and said it could go as low as 20.1 million in 2040.
Koppelman is quoted in the latest report as saying the numbers are "reasonable, possibly even conservative."
Koppelman acknowledged that it is difficult to predict how many riders the California bullet train would serve because the state lacks a developed conventional rail network. In contrast, France and Japan were operating conventional rail lines at or near capacity before introducing high-speed trains along the same routes. In those cases, planners needed to estimate only how much the new service would increase ridership.
Koppelman said he is among a few consultants in the transportation industry with expertise in travel forecasting. As such, he said, it would be difficult for the high-speed rail authority to hire anyone for his position who does not have past business relationships similar to his.
Kenneth A. Small, professor emeritus of economics at UC Irvine and a member of the ridership review panel, agreed, adding that Koppelman "has shown no coziness toward Cambridge Systematics. He's objective and fair. He is completely professional about it."