An artist rendering shows the interior of the planned Purple Line commuter trains in Maryland. (Purple Line Transit Partners)

Federal transit officials have determined that Metro’s safety problems and declining ridership would not have a significant enough impact on Maryland’s planned light-rail Purple Line to warrant updating the project’s environmental study, according to a court filing late Friday.

U.S. District Court Judge Richard J. Leon is expected to consider the filing in deciding whether to dismiss a lawsuit that Purple Line opponents filed against the project on environmental grounds. Major construction on the 16-mile light-rail line between Montgomery and Prince George’s counties, which had been scheduled for late October, cannot begin until the lawsuit is resolved.

Leon ruled in August that, as the plaintiffs had argued, the FTA erred in not considering Metro’s declining ridership when it approved the Purple Line’s environmental study in 2014. Leon ordered that the project’s review be redone — a process that would have added months of delay.

However, after the FTA and the Maryland Transit Administration objected, the judge backed off that decision in November. The FTA had to consider any impacts of Metro’s decline, the judge said, and then determine if it warranted a supplemental environmental analysis.

The Purple Line would be operated separately from Metro, but four of the light-rail line’s highest-ridership stops would be at Metro stations on the Red, Orange and Green lines.

Because 27 percent of a Purple Line’s ridership is expected to come from people transferring to or from Metro, opponents said Metro’s declining ridership called the Purple Line’s ridership forecasts into question. The state relied on those forecasts in its environmental study when it chose a light-rail line over a less expensive rapid bus option.

Metro ridership has fallen 12 percent since 2010, with 100,000 fewer trips per day.

In the filing late Friday, the FTA said the area’s population, employment and traffic congestion in 2005 would have supplied enough demand for a Purple Line to reach 80 percent of the 69,299 trips that the Purple Line is projected to reach on average each weekday in 2040.

If Metro ridership rebounded in 2018 — after the rail system’s SafeTrack repair program is expected to be completed — and grew at the rate initially projected, the Purple Line’s ridership would reach about 66,766 per weekday — about 3.6 percent less than initially estimated, the FTA said. Under the most pessimistic scenario — one in which Metro suffered a “long-term, sustained decline” in ridership — the Purple Line would have 13 percent fewer riders than projected, or about 9,000 fewer trips daily.

Even if the Purple Line had no Metro riders — removing Metro’s problems completely from the Purple Line ridership analysis — the light-rail line would have about 50,000 weekday riders in 2040, the FTA said.

With 50,000 riders under that “most extreme scenario,” the FTA said, the Purple Line would still be “one of the most robust light-rail systems funded by the FTA in recent years.”

Because the line’s construction footprint would not change, the FTA said, reduced ridership would not affect its environmental impacts. The line also would continue to meet its stated purpose of improving east-west transit in the Maryland suburbs, the agency said.