Becuase of the different operating conditions in the eastern mountains, driver diameters tended to be smaller than mid west and western locos.
Rockies may be taller, but the Appalachians are in many ways more rugged per mile, requiring more and sharper curves and steeper grades to traverse.
This created a need for locos that could manage constant moderate speed under all conditons rather than highest possible speed on relatively straight track.
Larger drivers mean a larger rigid wheelbase which ultimately reduces maximum speed on curves and reduces torque on grades. Other east coast Northerns had similar driver sizes.
Keep in mind that this would not have been a fast locomotive on any railroad other than N&W, and indeed was anything but a joy when built without the Timken lightweight rods and bearings. But the real secret was not that the engine was equipped with roller bearings (the UP FEF-3s did not have roller rods and were reliable and fast) it was that Voyce Glaze did the balancing and did not screw the pooch like the AAR-following people who balanced the ACL R1s and New Haven I-5s.
Indeed the issue with sustained high speed was more or less exactly where you’d expect to find it: valve tribology at high machinery speed combined with crazy high superheat level not encountered in N&W practice. But over 112mph from 70" drivers with a boiler large enough to sustain the necessary mass flow is not to be laughed at.
PRR advised larger drivers (at least 77" as I recall) but a moment’s reflection on how tall the result would be (and how top-heavy it might behave!) will tell you why there was no piston R2 class there.
When did RF&P ever have a 4-8-4 with an 80" wheel? I thought all the classes were 77"… not that that was inferior. These were really all-round the best-looking of 4-8-4s (in my opinion).
(Maybe I’d see more of you if I visited the MR Forum more often, but honestly there’s usually very little there to interest me.)
I just checked the book on the RF&P’s 4-8-4’s, God forbid I should call proud Virginia locomotives “Northerns,” and indeed all three classes had 77" drivers.
Incredibly handsome engines too, it’s a shame none were saved. That fact alone mystifies many in the Richmond area who remember, considering the pride the RF&P showed in those locomotives.
Why not separate out LV as they did the same experiment with the same general result with the last 5 Wyomings?
And the C&O had Greenbrier drivers at 72" about as long as the NYC had the Niagara’s at 75" – I think the early ones were 74" by 1947 and the later ones followed delivered at that.
And solid financials at the time as well. No poverty forcing their hand like some roads like the Rutland faced that were saddened to see the passing of an era, but just couldn’t justify donating several thousand dollars worth of scrap metal as a memorial to the steam age.
And late retirements for at least some of the RF&P’s fleet, which makes it perhaps even stranger. Back in say 1951 when you’re sending your steamers to the torch and steam is seemingly still everywhere in the region, it’s easy to see how it might’ve been overlooked. But 1959 was when their last 4-8-4’s went off the roster after several years in storage.
That’s a point where the steam fleets for most Eastern Class 1’s were already a thing of the past and the world was devoid of examples of important classes like New York Central’s fleet of Hudsons, any steam power from several historic lines like the Erie, etc.