If early 'American' type locos worked fine and engine pulled the amount of cars needed, what was the reason slide valve was replaced with Walshaerts, Stephenson, Baker, et al, valve gear ? No complaint, of course....
Efficiency, with that they could haul more, by hauling more the company could make more profit. Other than that it's because we humans always strive to improve things. Why DCC?
The chief reason was that they could not be used with super-heated steam. They had poor lubrication to begin with, and were prone to excessive wear. However, when super-heaters came on line, both the lubes used up until then, and the valves themselves, became distinct liabilities to the bottom line. Enter the widespread use of spindle valves and Detroit hydrostatic lubricators, and also the Nathan mechanical lubricators. If you look at closeup videos of steamers running along a highway, and the camera offers a good view of the valve rod and valve cylinder, you'll see a red T-handle rotating in concert with the valve motion and union link on Walschaerts valve gear, or maybe even the crosshead...it varied according to the locomotive. That rotating T-handle, which you also see the engineer or fireman turning by hand at stops, is the manual actuator for the mechanical lubricator. When it rotates with valve motion, it rotates only about 15 degrees at a time via a ratcheting mechanism. Note that most locomotives had two lubricators, but also a hydrostatic lubricator. The hydrostatic was for the valves and pistons using a special steam oil. The mechanical lubricators lubed the front truck bearing, the crosshead, and the wedges and bushings on the main driver axles, as examples of where that lube was used on the locomotive. In some case, lateral motion devices also needed lube.
