The first map shows the railroad circa 1934-1947 after the line from Bates to Prairie City was abandoned. This extension went over a third summit (Dixie) and required a switchback on the west side. This part of the line didn't see much traffic - passengers, mail/express, supplies and outbound stock since this was the railhead for a large region. The completion of the Oregon & North Western to Seneca created a better option for shipment of stock and improved roads got the rest. Formal passenger service end in 1937 but you could still buy a ticket to ride in the caboose. By 1947 the train was only making three round trips a week bringing rough saw lumber from the Bates mill to the planer at So. Baker. The railroad replaced the mainline haul with trucks in 1947 while #101 continued to move outbound finished lumber in standard gauge cars over the three rail until the end in 1961(?).
There has been speculation as to what the Sumpter Valley would have looked like had they either continued on to Burns or beyond Prairie City, and had been able to hook up with the NCO (assuming it ever made it past Lakeview). Not sure it would have survived long as a narrow gauge, and even if they had standard gauged the line there is hardly any traffic available. The mill at Prairie City has been closed for years, and their sister mill in John Day (tot he west) is scrapping out leaving one other small mill still in operation. There are no mills in or close to Baker City, Sumpter, Burns/Hines, Seneca and much less in Prineville. Lakeview is doing good business - one mill left, another small mill just built in Alturas, and perlite coming from somewhere north of Lakeview. There just isn't anything in the way of local traffic left along the former SVRy or the NCO for that matter.
We have speculated what if SVRy hadn't been so quick to dispose of the railroad and had mothballed it instead. Their lumber trucks took a beating and weren't quite up to the task, but it is not believed the owners would have continued the railroad much less dieselized it.