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Bay Area Rapid Transit's (BART) board yesterday approved the purchase of custom-made rail dampers to help cut the noise level on a section of track between its Balboa Park and Daly City stations in the San Francisco area.Made of steel and rubber, the dampers clamp onto the rail every 30 inches and help lessen the vibrations that cause a screeching noise when trains travel over the track, BART officials said in a press release. The board approved the purchase of 4,000 dampers to cover 3,000 feet of track between Glen Park and Daly City and 840 feet of track near the West Oakland Station. Grants will cover most of the estimated $600,000 cost, agency officials said.Crews will install the dampers near Balboa Park over Labor Day weekend, when BART plans to shut down a section of track for repairs as part of its rebuilding and rail replacement program. Installation at the West Oakland Station will occur a year later and won't require a track shutdown.Rail corrugation, which is caused by the way rail car wheels interface with the rail, is one of the reasons for the noise, agency officials said. When trains pass over corrugated track, there is vibration and resulting high-pitched screeching sound. Crews will use a rail grinder when the system is down at night to smooth the corrugation to reduce noise. The cost of purchasing and installing the equipment is about $1 million per mile, so the agency isn't yet fixing the entire system, said BART Chief Maintenance and Engineering Officer Tamar Allen. "Instead we are targeting the nosiest areas and will evaluate the success and the cost benefit of not needing to grind rail in those spots as frequently to determine how to move forward," Allen said.In 2012, BART installed dampers on a curved section of track on the aerial guideway between the North Berkeley and El Cerrito Plaza stations. This led to a "noticeable" impact in reducing noise and decreased the need to grind the rail from twice a year to once every four years, agency officials said.