These are components of a steam locomotive boiler. They are called fusible plugs. They are screwed into the area of the boiler above the fire where under normal operation they are covered with water. They comprise a gunmetal bolt which is hollow through the thread and with a small hole in the head. The hollow region is filled with lead. If the water level drops in the boiler below the operational level the lead melts. This then causes steam and water to spray onto the fire which extinguishes the fire and so prevents the boiler from damage by overheating. These two are being changed out as part of the annual boiler inspection. Reconditioned ones have replaced them. April 2024.