The next phase of overhead wire replacements on Metro-North's New Haven Line is scheduled to begin early next year along two busy stretches, but will require fewer service disruptions.

The work will begin early next year to upgrade the overhead wires in a six-mile section east of South Norwalk to near the Greens Farms train station and a five-mile section of track starting east of the rail bridge over the Pequonnock River to the Devon rail bridge, said Jay Young, project engineer for the state Department of Transportation.

Unlike ongoing work to replace catenary between Bridgeport and Southport, the project will require the continuous outage of one track in both segments of the project, rather than two, limiting the service impact of the project, officials said.

The work is part of a project begun in 1998 to replace the railroad's aged network of more than 178 miles of the overhead catenary power system, a project that has grown in price from a $300 million estimate in the early nineties to near $1 billion. Trains on the New Haven Line run on the overhead catenary system east of New Rochelle, but can run on a third-rail system from there west into Grand Central.

In late 2011, Metro-North and the DOT decided to speed up the completion of the catenary work by scaling back some other bridge replacements and improvements initially included in the scope of the project.

As a result, the work, originally scheduled to be finished in 2021, is now slated for completion in 2017.

The shorter time frame was accomplished by delaying the start of work to revamp two of the railroad's century-plus-old moveable bridges, the four-track Walk bridge in Norwalk, and the Saga bridge in Westport from 2014 to 2017.

"The accelerated schedule is based on a catenary-only project," Young said. "Previously, the catenary projects in Connecticut on the New Haven Line included bridge replacements."

An ongoing project to complete seven miles of catenary between Southport and Bridgeport is expected to be completed in December, after which Metro-North will spend two months completely replacing ballast, ties and rails in that stretch of track, Young said.

The catenary system uses a hydraulic arm atop railroad cars drawing energy from a wire charged with 14,000 volts of alternating current, which is converted to 700 volts of direct current to power the train.

The new constant tension catenary system is designed to be less vulnerable to malfunctions during hot and frigid temperatures that affected the hydraulic arm's alignment with the power cable in the older system, Young said.

The new system provides extra wire when cold weather contracts the wire, and tightens it when it slackens on hot summer days.

"By the end of the project the entire New Haven Line will have the state of the art constant tension system," Young said.

Jim Cameron, a member of the Connecticut Rail Commuter Council and a Darien commuter, said that it will be important for the state and railroad to be proactive in telling riders if any extended pattern of service delays will be caused by the work on an ongoing basis.

Last month, Metro-North introduced a new timetable to help accommodate track maintenance upgrades that were ordered after a May 17 derailment of a train in Bridgeport and collision with another that injured 70 people.

Arrival and departure times were shifted by up to 10 minutes for most trains as part of the schedule change, but the railroad could lessen the negative reaction to service disruptions by being clear about the need for improvements, Cameron said.

"There is work to be done and it is hard to change a fan belt on an engine that is running," Cameron said. "I think even commuters who may be upset with the delays in recent weeks would put a first priority in making sure their ride is safe."