Protesters arrive at the Capital South Metro station for the Women’s March on Washington on January 21, 2017 in Washington. (Photo by Jessica Kourkounis/Getty Images)

First of all, understand this: In measuring subway ridership on a given day, Metro can’t tell precisely how many people it carried. The transit agency doesn’t record names or create passenger manifests. It merely counts heads electronically — very often the same heads multiple times — and tallies ridership not in terms of individuals, but by “passenger trips.”

So when Metro says that during Saturday’s Women’s March on Washington, subway ridership for the day and evening totaled “1,001,613 entries” — the second biggest daily volume in Metro’s 40-year history — it didn’t mean that 1,001,613 people used the rail system.

Assuming that a vast majority of the 1,001,613 “entries” were made by riders who later entered the subway again, to get back to where they came from, it’s virtually certain that the number of people who used the system during its 19 hours of operation Saturday was closer to a half-million.

Saturday’s entries were exceeded only by the 1.1 million entries recorded Jan. 20, 2009, on President Barack Obama’s first Inauguration Day, Metro General Manager Paul J. Wiedefeld said in a statement Sunday. By contrast, Metro said, subway entries Friday, during President Trump’s inauguration, totaled 570,557 in a 20-hour period, from the system’s 4 a.m. opening until it closed at midnight.

Friday’s ridership also was lower than that of a typical weekday and lower than it was during Obama’s second inaugural, in 2013, when the Metro recorded 782,000 subway trips.

“Over the two days of Friday and Saturday,” Wiedefeld said, “Metro trains, buses and paratransit served well over 2 million passenger trips. … We can all feel proud of providing safe, reliable service for large numbers of riders over two consecutive days on a world stage. This success is especially impressive given the monumental challenge of sustaining such an operation over back-to-back days, along with the logistical challenges that come from national special security events.”

In light of the subway’s troubled history of breakdowns and other service-related problems in recent years, the system operated efficiently over the busy weekend.  “I am so proud of what our team accomplished,” Wiedefeld said. “We demonstrated to the region and riders from across the nation that we are capable of delivering world-class service.”