Residents are uneasy over potential health risks posed by silica. Open train cars filled with sand have raised alarm in St. Paul's St. Anthony Park neighborhood, where some residents wonder if the silica that comprises 80 percent of the unprocessed sand is safe. The cars, which often are parked in a rail yard sandwiched between homes and an industrial zone, are appearing on a more regular basis since an oil boom has increased the demand for the sand for its hydrofracking operations. BNSF Railway Co., which transports the sand, and Minnesota Commercial Railway representatives said the trains are carrying wet...