Actually the 50 wasn't that unusual at all considering it is out of the same class as the NCO/SPng engines and a sister to SP #18, and I believe the Tweestsie engines are the same or close. I will admit it takes a while to get used to the short smokebox, and I'm partial to whaleback tenders but it's not the best design for a woodburner.
Early American locomotive practice was to assign 4-4-0 Americans to passenger service and 2-6-0 moguls to freight service, often with interchangeable or very similar boilers. The Americans had less weight on drivers and less tractive effort, but the greater steaming capacity to tractive effort allowed it to maintain a greater percentage of power at higher speeds compared to the Mogul. The larger drivers of the time allowed lower piston speeds. As lubricants, designs and machining practices improved, the economical and maximum piston speeds increased allowing smaller drivers resulting in smaller cylinders.
Sumpter Valley only had two known Americans - the original construction engine (285/1st 4/1st 15) from the U&N was traded to the Eureka-Nevada in pat of a multi engine deal where E-N's heavier engines not suitable for their use were traded for SVRy's lighter engines. As part of the deal, SVRy received a 4-4-0 from E-N that had originally been a C&C engine. SVRy used it briefly as their 1st 10 before it was sold to Oregon Lumber Company as their 103. The preference for Moguls instead of Americans on the SVRY was for a very simple reason - there were few places an engine could open up and run fast but it had three summits with very steep grades and needed all the tractive effort possible.
The American design grew into the 4-6-0 Ten-Wheeler, of which 50 was the only SVRy one. The Mogul design grew into the 2-8-0 Consolidation which were still fairly rare on the SVRy. Up to this point the fireboxes either were located between the axles and frame (the longer distance between 2nd and 3rd drivers on moguls and ten-wheelers) or on the frame above the wheels. Once suspension designs were figured out, adding a trailing truck to create the Pacific and Mikado types allowed much larger fireboxes and even greater steaming capacity as the firebox could now be full width and behind the drivers, supported by the trailing truck. Buying the Baldwin Mikes 16 and 17 in 1915 was a huge jump in weight and power for the Sumpter Valley and probably resulted in substantial track improvements.
In all probability instead of buying another Mike SVRy chose to buy a new engine specifically suited for hauling two or three passenger cars between Baker and Austin, and since I think the new cars being considered had steel frames they were probably a little heavier.
Looking at the information I could find on Tonopah 4 which became SVRy 2nd 4 and is the one listed in 1916, it also had either 44 or 45" drivers and its tractive effort calculates out to about 16,000 pounds. By comparison 50 had 18,600 pounds tractive effort, and would be rated at about 130 tons on 4% versus 110 tons for the 4, allowing the 50 to handle a heavier passenger train at higher speeds due to greater boiler capacity. 50 appears to have handled plenty heavy trains working the logger extra, and photos also show it on the passenger trains. 19 & 20 were considered "slippery" and SVRy soon turned down their boiler pressure and they were also used on the passengers, and even now 19 wants to slip and doesn't like low speeds. Note that the Baldwins were held onto until after the war, yet 19 & 20 were sold before the war right after the mallets arrived.
