MattA
The N&W Class J's starting tractive effort (STE) is the subject of much discussion, but I prefer to look at it this way. The J was designed with relatively large cylinders, such that its calculated STE would be somewhat high. Under poor rail conditions, it certainly could NOT get all that force to the rail. However, keep in mind that the J's daily job was to accelerate trains from frequent stops to track speed relatively quickly, move 15-17 car trains over mountain grades and start them on these grades when necessary, and accelerate from 25 mph slow orders to 50 mph repeatedly. An 17-car train doesn't not require 80,000 lbs STE to get underway on N&W's grades. So the J's large cylinders came into play above 25 mph, where slipping would be much less of a problem than when starting. They gave the locomotives almost breath-taking acceleration in the mid-speed ranges. I witnessed this phenomenon in 611's cab (June 1986), and it's not imaginary!
As reference to the extreme low speed and of things, I recommend looking at the video on youtube showing 611 barely making it over the summit at Linden, Virginia, on the Manassas-Riverton line during an excursion in 2016. But make it she did, at about 1 mph with one pop valve lifted, indicating that even under lousy rail conditions she could still hold on and get a 20-car train over the top of a 1.6% grade on something like a 2-4 degree curve!
