2. What was the explosive device ? The Coal Torpedo (aka Coal Bomb); an explosive device disguised as a lump of coal. The coal bombs would be planted in the coal piles used to fuel Union steamships and locomotives by a team of operatives working behind enemy lines. When a coal bomb was shovelled into the firebox , it would explode, resulting in the explosion of the pressurized steam boiler and the destruction of the vessel or locomotive.