Now thanks to Ronald Bailey I have an insight into what might have caused that accident on the railroad back in 1892. Ronald Bailey is the President of the Railroad Museum of Pennsylvania Advisory Council, an avid railroad historian, and a licensed steam locomotive engineer. He was able to provide me with an understanding of the technicalities of an explosion such as the one that Engine No. 563 experienced.
Here is his technical explanation of the workings of a pre-1900 steam engine and the possible causes of the type of explosion that killed William Cowhey and the rest of the crew and passengers of Engine No. 563:
A locomotive boiler explosion is usually caused by one of two factors - a defect in the boiler shell or the crew allowing the level of the water in the boiler to drop below the crown sheet of the firebox. In the 19th-century locomotive boilers were commonly made of cast iron rather than steel because steel was far more brittle. Cast iron was stronger than steel and more pliable without tearing. Unfortunately, cast iron could contain unseen inclusions and defects that could cause a failure. As metallurgical techniques improved better types of steel were developed and by the 1890’s almost all locomotive boilers were constructed using steel. Boiler defects were rarely the cause of a boiler explosion in locomotives built after 1900.
A locomotive boiler consists of a cylindrical boiler shell, fabricated of thick cast iron or steel. The boiler shell is thick enough to be able to resist the force of the steam, which, depending upon the locomotive, varied from 125 to 300 pounds per square inch. In 1892 a common boiler pressure was 160 to 180 pounds per square inch, and shell of the boiler was built to safely contain at least twice that pressure.
The added problem, however, is that the fire in a firebox burns at a temperature between 1,800 and 2,600 degrees Fahrenheit. This is hot enough to melt steel. In fact the only thing that keeps the fire from destroying the firebox of a steam locomotive is that the firebox is surrounded on all sides, except the bottom, by water, which absorbs the heat in the process of boiling and making steam.
Boilers were fitted with “try cocks” (three valves stack one on top of the other) that an engineer could use to check the level of the water. Most locomotives were also fitted with water glasses that displayed the water level.
As long as the water in the boiler was kept at a level that completely covered the firebox, everything worked fine. If, however, the water level was allowed to drop below the top of the firebox, which was called the crown sheet, the metal of the crown sheet would be subjected to the intense heat of the fire without any water to absorb the heat. This could cause the metal of the crown sheet to soften and begin to melt. As the metal softened the steam pressure could force the crown sheet to pull out of the thread of the staybolts. Without the support of the boiler shell through the staybolts, the crown sheet would deform and at some point of stress would crack or tear. This would suddenly create an opening that would allow all the steam and boiling water in the boiler to exhaust to the atmosphere. The almost instantaneous expansion of the steam from 150 to 180 pounds per square inch of atmospheric pressure produced a terrific force, which was usually violent enough to rip the firebox sheets and tear the entire locomotive boiler off of the locomotive frames. The effect was pretty much like a rocket taking off and exploding. Boilers were sometimes hurled hundreds of feet away.
In the accompanying photograph, the remains of a locomotive after a boiler explosion are visible. The boiler has been torn from the locomotive frame, thrown through the air, and has landed upside down in a nearby field. The rear of the boiler, with the remains of the firebox, is on the left. The smoke box has been deformed and flattened. Strips of lagging are visible where the sheet metal jacketing has been torn loose.