WASHINGTON — As hundreds of commuters emerged from Amtrak and commuter trains at Union Station on a recent morning, an armed squad of men and women dressed in bulletproof vests made their way through the crowds. The squad was not with the Washington police department or Amtrak’s police force, but was one of the Transportation Security Administration’s Visible Intermodal Prevention and Response squads — VIPR teams for short — assigned to perform random security sweeps to prevent terrorist attacks at transportation hubs across the United States. “The T.S.A., huh,” said Donald Neubauer of Greenville, Ohio, as he walked past the...