The inauguration of Barack Obama on January 20 is expected to draw huge numbers of people, perhaps setting a record for a presidential swearing-in. 

Estimates range as high as three million, but even half that number would be the largest crowd in inauguration history.

The Washington, D.C. transit system, called Metro, will be the prime people-mover on Tuesday. But, depending on the turn out for this year's inauguration, Metro may be inundated to the breaking point.

The noontime inauguration of Barack Obama as President of the United States will be a historic moment.  And Washington, D.C. is bracing for it. 

Mr. Obama's swearing-in is expected to draw more people to the Capitol grounds and the National Mall than have ever congregated before for an inauguration. How will they get there?  

For many, on Metro, Washington's subway. Opened in 1976, Metro covers the District of Columbia and extends into parts of two surrounding states, Virginia and Maryland. 

Metro has more than 900 rail cars, each capable of carrying around 150 passengers when packed to standing-room-only.

But Metro spokesman Steven Taubenkibel says the subway system can only carry a finite number of passengers. He stated, "The most number of people that we can carry in any one given hour in our system is 120,000 people per hour.  That is the maximum number."

With an expected inauguration crowd of at least a million, Metro plans to open the subway at 4 am, an hour earlier than usual.

Metro's Taubenkibel says he hopes this will prevent the expected inbound crush from becoming a human tsunami. "The difference is, in the morning, with people going there, we will have seven, almost eight hours, where people are coming in in waves," he said.  

Many Metro riders will start their journey to the inauguration from the outlying suburbs. Officials in Virginia and Maryland say they are girding themselves for the onslaught. 

Maryland Transportation Secretary John Porcari said, "There will be, or what is likely to be, record or near-record ridership on Metro that day."

Along with moving people, Metro is concerned about security. Officials won't openly discuss security measures which are expected to be unprecedented. But they say they are very mindful of past terrorist attacks on rail systems, such as in Madrid in March, 2004, and in London, in July 2005.

U.S. Secret Service Director Mark Sullivan says rail riders need to be part of the security effort. He tells Metro riders to, "Be vigilant. Be smart. And, be alert to things that don't seem right in your surroundings. 

But for many going to the Inauguration, the biggest concern after arriving at the event will be getting home afterward.

Metro's Steven Taubenkibel says that is when the subway system could become overwhelmed. He said, "When the swearing in ceremony is over, it could very well take several hours to clear the National Mall, to have all of those people come into our train stations."

Washington Mayor Adrian Fenty offers one solution for coming and going that may not be all that popular on what will probably be a cold, blustery winter day. Walking.