NEW YORK — The CTA is expanding a long-awaited tunnel extension that would stretch from the Chicago River to the lakefront near the intersection of North Avenue and Randolph Street in Northbrook, but the extension is expected to be far more costly than expected, costing $6 billion to $7 billion, the agency announced Thursday.

The CTS, which owns and operates the tunnels that will stretch 1.7 miles, said the $6.8 billion tunnel extension would be the first tunnel expansion in the nation and the third to be built under the CTA’s Red Line.

It will replace a 6.1-mile stretch of track between Cermak and Westport that opened in November.

The Red Line extension would connect the Loop with the North Shore Expressway and to the Lake Shore Drive, where the CTS plans to build a new station.

The new line extension would have a capacity of 7.1 million riders a day.CTA Chairman John Zimmer said it would be a “boring” project that could take as long as two decades.

“There’s a lot of money in the tunnel, and I think you could say there’s a significant amount of work that has to go in the tunnels to do that,” Zimmer said.

The $6 million tunnel extension will connect the Lakeview Drive with the Chicago, which is the last major road connecting the city to Lake Michigan.

The CTA said it plans to spend $632 million on the extension.

“The CTC will spend a great deal of money to extend the Lakeline,” Zimmer added.

“It’s a great improvement, but it will be a long time before we get it to Chicago.

It’s going to take a while.

We’ve been working very hard to make it a great connection for the people of the city, and we are confident it will do so.”

The tunnel extension is the latest in a series of improvements planned for the Red line extension.

It is part of the $1.4 billion project that will connect Lake Michigan to the Red and Blue lines, including a new CTA station at the West Loop.

The Red Line project has also been stalled for years.

The project was supposed to be complete in 2023 but has been delayed by delays and cost overruns.

The cost of the tunnel extension has not been determined.

The cost will depend on the cost of a new track to connect the Red lines to the CTC.