A rendering of what a light-rail Purple Line train would look like running through the University of Maryland campus in College Park. (Maryland Department of Transportation)

A U.S. District Court judge took Maryland and federal officials to task Wednesday for ignoring Metro’s maintenance problems and falling ridership when they estimated how many people will ride the Purple Line, a shortcoming the judge attributed to a “cavalier attitude” that he found “troubling” for a $2 billion transit project that will connect to the Metro system.

But Judge Richard J. Leon didn’t seek an answer to a key question that critics have pressed for years: How, exactly, did the Maryland Transit Administration (MTA) come up with those ridership numbers in the first place?

Purple Line opponents say the MTA hasn’t disclosed the assumptions it entered into the computer model that calculated how many people will ride a Purple Line, both in its opening year of 2022 and decades into the future. The ridership projections are important both in determining: 1. Whether the cost-per-rider for construction and operations will be worth the price of a light-rail line over a cheaper option, such as a bus system. 2. How much the state will end up paying in the long run, both to pay off the construction debt and to subsidize the train service, because transit fare revenue doesn’t cover their operating and maintenance costs.

But modeling experts say it’s impossible to know how realistic the calculations are unless you also know what assumptions were made for the inputs into the model — and the MTA, critics say, hasn’t shared that information with the modeling experts that opponents hired to double-check the math.

Such assumptions include how far people are willing to walk to a rail station, how much gas is expected to cost in the future and how quickly new homes and office buildings planned around future light-rail stations will be built. Experts point to the potential problem of “optimism bias” — the idea that people who work on a project for years can become so personally invested in its success that they pick the more optimistic assumptions.

One reason Purple Line critics have focused on the ridership projections is because they have fluctuated, sometimes dramatically, over the years and seemed to follow the views of whoever lived in the Maryland governor’s mansion. The ridership numbers shot up by 45 percent in 2008, after Martin O’Malley (D), who championed the Purple Line, took over from his predecessor, Robert L. Ehrlich Jr. (R), who had favored building the Intercounty Connector toll road.

Maryland transit officials have said the numbers changed as they continued to refine the model and that it’s not unusual for such projections to fluctuate as a project’s design becomes more detailed. Maryland Transportation Secretary Pete K. Rahn — whose boss, Gov. Larry Hogan (R), was once skeptical of the Purple Line — said last year that he was “comfortable” with the numbers and noted that they had passed muster with the Federal Transit Administration. The FTA, Maryland officials say, wouldn’t award highly competitive federal construction grants to the Purple Line — in this case nearly $1 billion — if it didn’t think it was a sound investment.

Maryland taxpayers, whether they’re for or against the Purple Line, won’t be able to check for themselves. When the town of Chevy Chase, which opposed the line, hired a New York engineering firm to scrutinize the ridership forecasts, the MTA provided data files that were indecipherable without a specific software. The MTA said the town would have to buy the software from its consultant. The town said the software was too expensive and would take too long to get.

The plaintiffs in the federal lawsuit raised the issue as one of many arguments, and asked the judge to require the MTA to make the software available. Without it, the plaintiffs argued, the state “undermined” public scrutiny of its Purple Line study.

Lawyers for the state argued that, unlike the town, the plaintiffs didn’t ask for the ridership modeling data during the Purple Line study’s public comment period and that government agencies are not legally required to provide commercially available software. The MTA, the lawyers said, responded to technical questions and had an “extended dialogue” with the town about the Purple Line modeling process.

The public will have to take the government at its word.