The way the Metro works is that it has stations within walking distance of the National Mall. The authorities have encouraged people to take the Metro but by now, the areas within walking distance of the stations are full. Further, the Mall is at capacity so anyone getting there without a ticket will be stationed at least a mile away.
Sadly, a person was struck by a Metro Train at Gallery Place (not far from the National mall) and that means that the Red Line is not up and running in either direction. Not good. They are counting on all the lines to move people around and the Red Line is a busy one that brings people in from the northwest, loops through downtown and heads back to the north. Or it can go the other direction. I expect it will be up and running by the time the ceremony ends. Let us hope there aren't any other accidents.
By 9:30, the Metro had transported 409,828 people.
If you look on TV, note how the flags are unfurled. There is a fairly brisk wind. With the huge crowd, I suspect most people have a wind break made of human bodies.
They are now predicting over two million people. That is huge. Washington is not, geographically, that big.