When I worked for the SP, there was one industry, a refinery, which moved 120 ton loaded covered hoppers full of coke down a grade like this for about 3/4 - 1 mile, as many as 6 loads at a time. We charged the brake pipe to 110 psi, tested the brakes, and then waited for the system to recharge before starting down. If you kept the speed below about 7-9 mph, you could control your speed with the jeff with about 1/2 of a full service set AND stop for the closed gate at the bottom of the grade. It's all a question of not exceeding that critical speed where braking effort ceases being able to stop your train. The trick is to figure out exactly what that speed is.