A little hard on the rig...
The sand/gravel long runs don't total the tractor... at least the one I saw well into the run.
With the traditional runaway truck ramp and sand/gravel bed, the truck has a very high chance of turning over on it's side. They told us in school (80's) that you can expect to go over on your side, and to make sure your seatbelt is on tight for "the ride" and the sudden stop at the end. I've never had to use one (truck ramp), but there was one time I did need one and there were no ramps in that area at the time. They have long since redone the pass, and added additional ramps.
Coming down Raton pass with no brakes, late 80's, by the time I knew I was in trouble, I was already past the last ramp. Long story short, half my brakes were disabled on the rig (contractor error) and vehicle maint tech didn't check them (but said he did to the NCO in charge when asked. In the post incident investigation, he admitted he just "assumed" they were fine). Military vehicle maintenance at that time was responsible for checking/adjusting brakes, as it was considered a "maintenance task", not a "drivers task". In the civilian world, most drivers check and adjust their own brakes. Maybe some company fleets still don't, dunno about that.
Normally we drove very conservatively, so didn't "need" all the brakes until we hit Raton (this trip originated in Boulder, CO). Then they weren't there. The "speed up" was gradual, I was in like 5th gear at the top (start of the grade), full Jake brake, plenty of time to figure something out (so I thought). By the time I knew I was hosed, we'd passed the last ramp. Looking back now, with 32 years of hind sight, I should have taken the last ramp, I knew "something was wrong", but I was trying to not destroy a 30 million dollar mobile satcom missile warning system (the truck). If it had "gone over" it would have destroyed the phased array antenna system on the side of the truck, and those trucks were irreplaceable, only 6 in the world (at that time).
We were instructed in driver's class that if we were ever in that situation, no brakes, runaway truck, to not allow the engine to over rev and blow itself up. Statistics from truck crashes on grades pointed to a much higher crash rate if the steering was gone. So keeping the engine alive meant you had power steering to steer the rig on the road. So we were instructed to as a last ditch choice, to slap the transmission out of gear and coast down the grade in neutral. At least we would be able to steer (some chance is better than no chance).
As I was slowly upshifting gears, with full Jake brake on, coming down the pass, I was able to keep it at a fairly mild road speed until I ran out of gears. Once the engine tach topped out in 9th gear, I did as we were instructed and slapped it to neutral. The rest of the trip down the hill was much more exciting, as we got some impressive road speeds (I'm sure I was well above 100 mph for a while). But by then, most of the big curves were behind us and I could just focus on keeping the rig on the pavement.
Made it to the bottom in one piece, still on the pavement, finally got it to roll to a stop, took miles and miles of relatively straight, flat road. Got the truck to actually come to stop by finally having the kid riding shotgun to lean out the cab door on his side and throw a set of wooden tire chocks under the truck. It took the lead convoy pickup about 15 minutes to catch up to us after we stopped, and he had a max speed of 80 mph in a gutless 6.2 L GM diesel dually. He'd been giving it all it had, lights and siren blaring, trying to "stay with us".
As an aside observation, when you're coming down a hill in a semi, at over 100 mph, riding the fast lane, flashing high beams and laying on the air horn, people tend to actually get out of your lane and let you by.
Good times.
