These are called tail rods. They were very common in large stationary steam power. Basically with a tail rod both ends of a piston rod were supported independently of the cylinder, so the only pressure on the cylinder wall is that of the piston rings. This greatly reduced wear of the cylinder wall. With large stationary steam engines, such as the 68" diameter piston of our Tod engine, which easily weighs a couple of tons, that weight bearing down on the cylinder wall would wear it out in short order.
The european practice of using tail rods on locomotives helped to prolong the life of their locomotives. In the US we didn't have time to bother with such frills, we as a nation were too busy laying tracks down every nook and cranny of the countryside. However, some early locomotives, like the first PRR K4, the 1737, had tail rods when built.
The photo is of the tail rod on our 34" high pressure cylinder.
The Tod Engine
todengine@woh.rr.com
