Ed
This apples, oranges and bananas business can be fun. It seems that the N&W had a fairly unique situation with superpower steam, an intensive utilization plan and a passenger schedule that included modern, roller bearing equipped, lightweight, streamlined trains.
What kind of total accumulated mileage did the J's amass before they were retired?
Only a few operations had similar conditions with steam locomotives as power for any significant length of time. The fast Chicago-Twin cities trains of the Milwaukee would fit and the C&NW might too. But the CB&Q went diesel early for that service. The SP daylights would fit the bill as well as the NYC Hudsons and Niagaras in their various services.
Perhaps a UP, SP or ATSF fan could chime in with accumulated mileage or utilization plan information from those roads.
For the northern transcontinentals their aren't any close comparisons to the N&W that I can think of in the steam era.
The Milwaukee electrified the mountain crossings starting in 1915. That made intensive steam locomotive utilization difficult on the gap between Othello, Wa and Avery Id. There were very few super power steam locomotives ever assigned to the gap and the S-3 Northerns only saw freight or secondary passenger service during the Korean war. When the lightweight, streamlined Olympian Hiawatha came along on a fast schedule in 1947 it was dieselized.
The Great Northern never purchased a new passenger locomotive that would qualify as Super Power. Their only modern passenger power, the 28 P-2 Mountains, the 6 S-1 Northerns and 14 S-2 Northerns were all built between 1923 and 1930 with plain bearings. To get an idea of how different the railroad world was in the "standard" era of the 1920's when the P-2s were delivered they were given to assigned crews. That limited the locomotives to about 4800 miles per month. But even in the 1920s there were events that pointed to what was possible. In 1925 GN 2517, one of the P-2 Mountains, hauled an 18 car silk train from Seattle to St. Paul without change in 52 hours and 35 minutes. After servicing in St. Paul it returned to Seattle on the Fast Mail with a scheduled time of 47 hours and 30 minutes. In recognition of its nearly 4000 miles of running in six days the 2517 received the title Marathon and her tender was so painted for many years.
Some of the P-2s and all of the Northerns received roller bearings in the mid 1940s. But by then the E7s were already on the scene and taking over part of the Empire Builder and Fast Mail schedules. When the streamlined '47 Empire Builder went into service on a faster schedule it was fully dieselized. Steam continued with the old heavy weight equipment as the resurrected Oriental Limited for a few years but by 1950 all regularly scheduled mainline passenger trains were assigned to diesels.
The NP was more progressive in its steam locomotive purchases. Of the six classes of NP Northerns only the 12 pioneer A class locomotives of 1926 did not have roller bearings. However, the Timken locomotive spent much of its career in a locomotive pool with the As and may have been utilized less intensively than it could have been if it was part pool of more modern locomotives. Early in it's career the Timken worked passenger trains on the Rocky mountain division. For the latter half of its life the Timken locomotive worked out of Seattle over Stampede Pass and on passenger and fast freight service to Portland. Seattle-Portland is a shorter run than Monroe to Bristol. If I recall correctly the Timken locomotive accumulated about 1.5 million miles by the time it was retired.
The A-2 through A-5 classes were all of the same basic design and very modern engines. The A-5s were second in weight among all Northerns due to war time material restrictions. Only the Santa Fe 2900s were heavier. But the same WPB restrictions that added to the A-5 weight also kept them in fast freight service on the eastern end of the system. The A-2s also saw considerable freight service during the war. I have never seen a total mileage figure published on the A-3s or A-4s. It might not be too impressive since they were not yet 10 years old when diesels began to arrive. Once that happened steam assignments were made to maximize diesel utilization. In fact the last regular steam passenger service on the NP lasted until 1956 specifically to avoid a diesel layover.
In 1957 the NP implemented a passenger diesel utilization plan that remains a remarkable achievement in efficiency. The locomotives covered over 9000 miles in 15 days or over 18000 miles per month. To do that they made two St. Paul to Seattle round trips, one Seattle to Spokane round trip, two Seattle to Portland round trips in passenger service and one Seattle to Portland round trip on a time freight. In 15 days the locomotives would cross Mullan Pass twice, Homestake Pass Twice, Bozeman Pass and Evaro Hill four times and Stampede Pass six times. They would also encounter significant curvature along all those mountain routes as well as along the Green, Yakima, Clark Fork and Yellowstone Rivers and the sawtooth 1% between Glendive and Mandan. While several of the NPs passenger F3s went to GE as trade ins in the 60s a few actually made it into Amtrak service in the early 70s. Those would have accumulated impressive total mileages under very challenging conditions.
Fix up that J and send her out here. If we can have a Montana Daylight why not a North Coast-Powhatan Arrow? And then we could see what a J could do in real mountains and not just those little hills back east. 
