Per Robert Macy:
"Engine Braking Prohibited", way out in the wilds, no one around.

Plus, the slope was so gentle? Maybe for the trucks?

They're *really* loud.... as in you don't want tb within even
miles of the noise - especially, I'd think, on wide-open country
without a lot of trees/hills to mitigate it.

As far as the gentleness of the slope goes, I don't have any
experience with heavily-loaded trucks, but I'd guess that if any
braking at all is required, the first choice from a cost-savings
perspective would be the compression brake, since it doesn't wear
out any consumables.
--
Pete Cresswell