Next month, Metro-North Railroad crews will finish the replacement of worn-out overhead catenary wires that power New Haven Line trains between Courtland Avenue in Stamford and South Norwalk, said Dave Willard, the railroad's assistant director of structural engineering.

Work will begin next fall to upgrade the wires along the next 6.8-mile stretch of track between South Norwalk and Southport, a task that will be finished in 2015, depending on funding, according to Willard. Another stretch of overhead wiring from Greens Farms to Bridgeport currently being overhauled is scheduled for completion in 2012, he said.

The pace of the entire project, which began in 1998 and is now about 40 percent done, is deliberate by necessity, Willard said. It requires regular closure of the track below where wire replacement is being done, in addition to additional track closures during off-peak periods to allow for the work to continue, he said.

Trains on the New Haven Line run on the overhead catenary system east of New Rochelle, N.Y., but can run on a third-rail system from there, west into Grand Central, according to the railroad.

"If a train breaks down on an adjacent track, we can't just flip a switch to move it onto another," Willard said. "It has been a constrained railroad for quite a while, with long-term track outages in one location or another for decades." The cost of the project, originally conceived in the early 1990s, has grown from an original estimate of $300 million to $878 million because of the additional replacement of 19 railroad bridges and more than 178 miles of overhead catenary wire and other infrastructure, Department of Transportation spokesman Judd Everhart said.

The final phase of the project is scheduled to be finished in 2020, with the completion of a section of catenary between Bridgeport and Stratford, which will include the completion of four rail bridges, Everhart said.

"It is important to note that the catenary replacement is a component of a larger program to renew bridges, as well as catenary, on the New Haven main line," Everhart said. "It is not a stand-alone project." This fall, crews have worked from 10 p.m. to 6 a.m. on the wires between Courtland Avenue and South Norwalk, as well as worked on the railroad bridge over Route 1 in downtown Darien, Willard said.

Work on the project from the state line to just east of Old Greenwich was completed in 2004.

In the catenary system, a hydraulic arm called a pantograph runs beneath a wire charged with 14,000 volts of alternating current, which is converted to 700 volts of direct current powering electric motors that turn the train's wheels.

Jim Cameron, chairman of the Connecticut Rail Commuter Council, said the rarely discussed work will have a dramatic benefit for commuters, though most are hardly aware of the issue of the catenary system unless they are stranded by a problem.

"It is invisible to the commuters, except when it doesn't work," Cameron said.

"But it is crucial to the operation of the railroad, and it is a pretty regular occurrence for wires to be ripped down, but when it involves new wire, it is probably the old cars and their pantographs that are to blame."

In 2008, the number of delayed, terminated and canceled trains because of overhead wire breakages and problems with pantographs fell to 160, down from 582 in 2007.

Cameron said retaining the overhead catenary system instead of considering a conversion in the corridor to a third-rail system was necessary because Amtrak trains operate on the overhead wire system.

The use of catenary will also leave open the potential to introduce higher-speed trains to the line in the future, Cameron said.

"I'm happy to hear the catenary project is moving along, and it is one of those things Metro-North and the DOT don't get credit for," Cameron said. "When it is finished, the railroad will be vastly improved."