Metro says the light rail line has recorded 100 million boardings since it went into operation in January 2004.
"When we looked at projecting ridership, we thought it would be four years from now before we'd meet the 100 million ridership mark. So we're beating it by a pretty significant margin."
Metro Interim CEO Tom Lambert says the 18 cars that carry passengers on the line have now travelled over four million miles. The agency has now purchased 19 new cars to suppliment the fleet. Two of those cars will roll out as part of the milestone celebration.
And Lambert hopes the free rides will encourage more people to climb aboard.
"Well its a great opportunity to go from the downtown Houston area to the Texas Medical Center and all the routes along the way, all lead to Reliant Stadium."
The transit agency is expecting huge ridership numbers when three new light rail lines go into operation in 2014.