Tim L is probably right on; I have noticed several big trucks with 3 - chime air horns on them, in particular a flatbed hauler with a set mounted sideways behind the sleeper. Also a dirt hauler w/ same set - up. Don't know if these horns are pirated from locomotives, or purchased legally from scrappers & / or Mfr. Recall that Woodinville Truss has a large facility along the Redmond spur; perhaps the flatbed is hauling lumber in / out.