I tend to believe WaPo about the Metro ridership numbers on Saturday
https://www.washingtonpost.com/local/2018/live-updates/politics/march-for-our-lives/metro-ridership-is-high-but-far-behind-womens-march/?utm_term=.7b1c6118afd6
Metro ridership is high, but far behind Womens March
Metros ridership numbers illustrate what many riders were seeing: large crowds.
By 1 p.m., more than 207,000 people had ridden Metro today, according to WMATA three times as many as a normal Saturday by that time. And heavy crowds were still making their way to the rally on Saturday afternoon.
By comparison, Metro reported more than 470,000 trips on the system by 1 p.m. on the day of the Womens March on Washington last year Metros second-busiest day in history.
Even though the numbers for the March for Our Lives arent as high as the Womens Mach, this is expected to be one of Metros top ridership days.
(snip)
((The local TV coverage, as the rally ended, showed quite a bit of space at the tail end of the rally as protesters moved away. Therefore, my own guesstimate of the attendance:
150K by Metrorail (remember the cherry blossoms are starting to pop out)
50K by bus
100K by car/taxi/other transport drops (plus some walk-ins)
HOWEVER, two local TV channels (plus C-SPAN, at least on radio) were doing live coverage of the rally. This may have cut down on the physical attendance in DC.))