The National Transportation Safety Board investigated three accidents that involved highway vehicles transporting bulk quantities of acetylene gas that occurred in 2007 and reviewed reports of a 2008 overturn accident of another vehicle. The vehicles, called mobile acetylene trailers, carried up to 225 cylinders that were connected by a manifold system and filled with acetylene. Two of the accidents occurred as the vehicles overturned on public highways, and two of the accidents occurred while the vehicles were being prepared for unloading.