Greanias appeared before the Greater Houston Partnership a couple of weeks ago and gave a glowing assessment of Metro's progress.
He pointed to millions of dollars in federal money for light rail expansion, along with an improved relationship with the federal government, greater financial transparency, and high ridership numbers.
"One-third of the folks working downtown today came in on some sort of Metro service. One-third of the people who work in the Medical Center came in on a Metro service."
But the work of running one of the nation's largest transit agencies will now go to someone else.
In his letter of resignation, Greanias told the Metro board it was an appropriate time to step down and create the next chapter of his career.
Rice University Political Science Professor Bob Stein says Greanias may be leaving because of the outcome of November's referendum on Metro's General Mobility Fund.
"And I think he feels this is a departure from the completion of the rail and I think he probably feels the board, and particularly the board's chair, and he did not agree on the future direction of Metro, particularly dealing with rail."
Under that referendum, Metro will continue sharing revenues with local municipalities. The agency will also use funds to pay down debt and run more buses.
But Stein says that means less money for rail, and Metro's board should probably look for someone with a background in running a traditional bus system.
"Clearly Metro's moved in that direction with the referendum, even before that, with contraflow lanes they had set up, with toll roads. But it's clear, you would not look for someone who has experience in doing rail."
Greanias leaves Metro effective December 31. The Metro board meets on Thursday to discuss a replacement.
George Greanias' letter of resignation: