Re: 315's main rods...

The right side appears to be a split brass. This is an earlier design that allows the brass to be adjusted for wear. There is a front a back half and a wedge on the back or the front (depending on design) to take up lost motion on the pin. Some railroads allowed a gap between the brasses so lost motion could be adjusted as needed. Other roads machined the brass halves so that when they were wedged tightly together the crankpin had proper clearance. Early side rods were fitted in this manner so all bearings could be field adjusted. It is a nice idea as long as you don't get the brass too tight on the pin or foul up the tram on the side rods by changing the distance between the brass centers as they are adjusted. Side rod adjustments disappeared fairly early.

Later years also brought about floating bearings on the big end of the main rod which is what #315 looks to have on her left side. This is just a bronze bushing - like side rods have - except they are not pressed into the rod. Rather they float and have clearance between the bushing and the rod and the bushing and the pin. Usually they are split pie fashion into three or four segments to prevent them from enlarging from pounding and sticking in the rod. This design is usually lighter and requires less counterbalancing. Additionally the wear is divided over four surfaces rather than two. The rod usually has a cast iron bushing pressed into the rod to give a better bearing surface for the bronze.