Today’s build was a pair of mirror imaged piston blocks. On a real steam train, the pistons drive the wheels. There is probably a term for it, on the model the turning wheels move the pistons, decorative not functional. Star Wars C3PO has a similar set up with his pistons, I remember him talking about it on Multi Coloured Swap Shop.
The push out diagram today had requested 8 small pieces, the piston head. These were on the plywood arranged in a 3×3 pattern, leaving the odd one for later, or so I thought. I also noticed the push out instruction had two more of these piston sections below the 3×3, but there were not marked on the Plywood.
I checked the rest of the instructions to discover these odd bits are not used. The little fiddly bits, it would appear that they include a spare or two. No mention of it so if one had snapped, I would have ended up trying to repair it or crafting my own.