You got down these grades in the days of steam with air brakes and a well balanced set, just like today, only with trains of that day which were generally of a lower tonnage and length, and at lower speeds. Before air brakes, trains were even shorter and lighter, and with a "swing brakemen" every 5 or ten cars, running across the roof, at the sound of whistle signals from the engineer, to tie hand brakes on each car, to "balance" the train descending grades.

Dynamics, if they are working, and you have sufficient axles for the train tonnage, generally allowed for descent at a higher speed, and with less stopping for recharging air(which often required holding the train until charged with hand brakes), to set retainers, and to cool wheels. Dynamics require less of a set to balance the train

I've only seen retainers set once in my 18 years of experience with second generation diesel electrics. That was on a beet train, I was firing on, with inadequate dynamics on both sets of helpers, and lead engines. In this case they also began to tie hand brakes, to control the train, as the air was released and the trainline was recharged. This was done up above where you drop down to old El Casco heading west toward Colton. The hoghead had a bunch of yellows and reds, causing the train to finally have to stop, and he was afraid he'd pissed the air away, and chose to make sure he had control of the train.... Which is always better than the train having control of you, as happened shortly after I left, with that poor piglet's runaway on the Palmdale cutoff.

Dynamic braking was/is a technology that allowed RR's to operate "faster, cheaper, safer".