Tight space constraints along a 4.3-mile light-rail expansion for the Sacramento Regional Transit District, or Sacramento RT, required engineers to modify plans and adopt new rail-damping technology to reduce noise in residential areas.

And crews with joint-venture contractor Balfour Beatty and Teichart must meet a strict time line to complete the work. Planned for more than a decade and delayed by funding challenges, the $270-million double-track extension of Sacramento RT's Blue Line to Cosumnes River College must be completed by September or risk losing federal funds.

On a section of track between Cosumnes River Parkway and Union House Creek, stakeholders, such as the city of Sacramento and the Sacramento Area Flood Control Agency, opposed planned sound walls because they could impede access, detract from aesthetics and impact pedestrian safety. To reduce noise without the walls, Sacramento RT originally explored using wheel dampers, but they were too bulky, says Darryl Abansado, director of civil and track design for Sacramento RT.

Instead, the agency opted to install 10,000 broadband-tuned mass rail dampers, using sandwiches of composite course steel and rubber plates, clamped together between the rail head and base. Carefully calibrated torque isolates noise and keeps vibration from attenuating out.

The dampers address "the sound at the source, rather than at the sound wall," explains Paul Kampfraath, founder of Netherlands-based Kampa International, the supplier for the technology. The Sacramento project will be the first full-scale deployment of the dampers in the U.S., he says. Transit contractors successfully implemented the technology in Australia, and San Francisco Bay Area Rapid Transit is currently in testing.

Kampfraath says the dampers also slow rail corrugation by changing the mass of the rail and impacting the wheel-rail interface.

Tests conducted in 2009 showed the rail dampers reduced noise by 2 to 3 decibels; however, the goal was a 5-dB reduction. Sacramento RT plans to implement acoustic rail grinding and track greasing, combined with regularly trued wheel maintenance, to reduce noise by up to 6 dB. Regular follow-up testing will ensure the measures reach their goals.

Sacramento RT also looked at adding skirts to the trains and installing mini 2-ft sounds walls, but both proved cost- prohibitive or had maintenance impacts.

Timothy Schmidt, senior associate director of Lockwood, Andrews & Newnam Inc. (LAN), the prime consultant and civil and track designer, says rail dampers are a good option to use when acoustic walls aren't viable. "If possible, home- owners tend to prefer walls," he adds.