CHICAGO (AP) — State and local officials announced a $1 billion project to repair Chicago's massive and increasingly dilapidated and slow public transportation system on Thursday.

Appearing at a Red Line train station on the city's South Side, Illinois Gov. Pat Quinn and Chicago Mayor Rahm Emanuel said the four-year project to fix Red and Purple lines and stations along the lines will create more than 2,700 jobs.

Most of the money — $646 million — will come from a $31 billion capital construction grant Quinn signed into law in 2009. In all, the state's share of the project will top $700 million. Another $255 million will come from the federal government, with about $44 million coming from the city of Chicago.

The city's train system is a source of major frustration for commuters, who have seen the trains get slower and slower in recent years. The Red Line, a north-south route that stretches for more than 20 miles, is by far the most used of all the Chicago Transit Authority lines, accounting for nearly 80 million riders, or 38 percent of the CTA's train ridership. The line, which in parts dates back nearly a century, is riddled with spots where trains are only allowed to travel 15 mph because of the condition of the tracks.

"We've got to do something about it," said Quinn.

Emanuel, who made improving the system's aging mass transit system a cornerstone of his mayoral campaign, suggested that because the Red Line is the "backbone" of the system, it was necessary to fix that first.

"The Red Line must be fixed," he said.

CTA President Forrest Claypool said after what he called the CTA's biggest construction project ever begins next year, all of the stations along the two lines will be repaired in some ways, with some undergoing major renovations. But he said he did not expect any of the stations to be shut down during the project.

Tracks will be replaced along the two lines, and Claypool acknowledged that the repairs may over the next few years slow down trains. But he said that much of the work would be done at night and would not be done during rush hour in an effort to inconvenience commuters as little as possible.