No one should ever be that close, but it's a worst case scenario within the realm of possibility (people do get themselves into danger sometimes, for example by wandering onto a railroad track when there's a train approaching). I don't think it's unreasonable to reserve the 10 on the 1-10 scale for 'loss of human life'.

  The train driver saw a man on the track ahead holding a cell phone to one ear and cupping his hand to the other ear to block the noise.