In the year 5,000,000,029, the population of the entire planet, including the Senate, were killed by an airborne virus caused by a mood drug. With the virus having spread beyond control, the Senate understood that it had very little time to act. In an effort to save at least some lives, the Under-city was completely sealed off in the hopes that the virus wouldn't be able to reach them. The last act of the Senate, before its members perished also, was to declare New Earth unsafe to the surrounding star systems and placing the planet in an automatic quarantine which would only lift 100 years later. The only people who survived were the Face of Boe and Novice Hame, the latter surviving in the smoke produced by the former. They later controlled the Motorway. Hame looked after Boe, who in turn controlled the computer systems. However, the skeletons of the old Senators still remained.
In 5,000,000,053, 24 years later, after the Tenth Doctor first arrived on New Earth, the Doctor and Boe worked together to save the people in the Motorway, who proceeded to repopulate the main city of New New York, and the Senate was reestablished. (TV: Gridlock)