I think it's an attempt to improve balancing, the 2 cylinders being 180 degrees apart would be in primary balance, apart from a slight rocking couple due to them being slightly ofset. However, as Tim says, the overhung crank looks like a recioe for disaster. I wonder if it was ever made?

Eric,

Probably a design intended to improve the power and smoothness of a locomotive by providing 4 cylinders without having to resort to a crank axel. Back in the days before metallurgy was fully understood and inspection techniques such as X-rays existed, crank axels frequently failed in a catastrophic way resulting in serious accidents. Robert Stephenson used double framing on his Planet type locos that had crank axels thus providing 4 bearings to each axel. If the crank axel fractured each half of the axel would still be supported by two bearings so the loco would safely remain upright on the track.

this loco is covered in my exhibited machinery of 1862 – 4 cylinders 4 cranks on a single driving axle – states that reciprocating and woking parts balance so that pistons on each side and all their connections move in contrary directions – comment by editor is that had it appeared 10 years prior it would have been important but now(ie 1862) use of counterweights achieves same object

the whole volume 420pp is a fascinating treasure trove to dip into