Adhesion......weight on drivers.....? I've watched many a high horsepower bb unit lose traction, catch again, and suck a knuckle.... That always seemed to be real good for maintaining "fluidity", too......as I seem to recall. Dying on the law, and replacing crews is always good for "fluidity", as I recall. And, let's not forget, having to cutoff power from other trains, to get stalled trains restarted.......tying up even more RR.

So, from my mountain experience on Beaumont and Cajon, I've found that you are almost always better to "walk" those heavy trains up a mountain with the big six axle units, with more adhesion, less slippage, more consistent drawbar pull, with sufficient "reserve" horse power per axle to start the train from a stop, anywhere on the ruling grade, and ideally enough extra HPT to maintain an optimal "speed" capable of keeping the RR "fluid".

My experience has never been that "minimum" HPT is ever the "optimal" HPT, when one factors in these other increased opportunities for things to go wrong, which eats up track time, crew time, goods in transit, and creates these unforeseen added costs...... But, this always looks so good on paper..... And, this does seem to be a lesson some RR's seem to reteach themselves every few years...as I recall.