The 1908 edition of COMPRESSED AIR journal ran a few articles about underground mining and the use of locomotive engines, coal mine vacuum cleaners, and other machines run off of compressed air. Different.
This is a Heisler Fireless (compressed air) engine stuffed and mounted next to the Hattiesburg, MS station. It was used at the Hercules Powder facility south of town.
You're correct. I added "compressed air" before reading the description. From the description: This locomotive was built by Heisler Locomotive Works as Hercules Powder Company #35 and served the Hattiesburg plant from 1946 until 1958, when replaced by a diesel unit. The Heisler is a "fireless" steam locomotive and was charged with steam from the plant boilers thus allowing extra safety when operating in a flammable environment.
Compressed steam was easier and cheaper to make than compressed air with the technology of the day. But in a mine, all that steam can make the air too humid to breathe, and leave the shaft a perpetual mudhole.