Sacramento Regional Transit District (RT) has begun construction on a $270m southern light-rail extension to Cosumnes River College in Sacramento County, California, US.

The 4.3-mile extension is expected to give commuters in Elk Grove and south Sacramento an alternative to Highway 99, as well as minimise congestion on Interstate 5. As part of the light rail expansion, RT has also started construction on two bridges at a cost of $22m.

Tracks will be laid on the bridges over Morrison and Union House creeks and Union Pacific tracks, west of the Franklin Boulevard and Cosumnes River Boulevard intersection, while the second bridge will be an intersection flyover at Bruceville Road and Cosumnes River Boulevard.

RT general manager Mike Wiley told The Sacramento Bee that the extension will provide a rail transit option at the doorstep of a city that is expected to remain one of the fastest-growing in the region.

"The extension is similar to light rail’s first suburban expansion seven years ago to Folsom, where park-and-ride lots are full and peak-period trains are standing room only," Wiley said.

The rail extension, which will run south from the existing Meadowview station, will provide a 30-minute ride between the college and K Street downtown. RT is also expected to provide limited-stop express services, saving five minutes of journey time.

The light-rail extension, which is expected in September 2015, was initially planned to open earlier before the economic downturn delayed proceedings.

The $270m project includes the costs of the bridges, as well as land purchases, flood control work, two pedestrian bridges, a bike and pedestrian trail along Cosumnes River Boulevard, habitat conservation work and a gas line retaining wall.

The US federal government has also agreed to provide $135m for the project and is expected to sign a funding agreement with RT later in 2012.

RT will utilise a combination of state transportation infrastructure funds and local transportation funds, including sales tax revenues.

Elk Grove vice Mayor Patrick Hume said that the city’s e-train will continue its freeway commuter bus service, as well as link to the college light-rail station.

"We are freeing up space for goods movement, we’re improving air quality, we’re providing options," Hume said.