Speed Creep on a 1.6% Grade With Dynamics at Full
You are operating an 18,000-ton, 135-car unit coal train with two head-end units and one DPU, descending a 1.6% grade for 12 miles. Speed has crept from 22 to 27 mph with dynamic brake at full and the independent brake released. Brake pipe is 90 psi, and the last air application was released 9 minutes ago. The engineer's air gauge shows main reservoir 140 psi and equalizing reservoir 90 psi. The next signal is 2.5 miles ahead, and the grade steepens to 1.9% in 1 mile. You have not used the automatic brake on this grade yet. The train is 8,100 feet long. The DPU is responding in sync. You have a 20 mph speed restriction at the bottom.
1. What automatic brake application do you make, and why not a full-service application immediately?
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Make a minimum reduction-typically 6 to 8 psi-not a full-service application. The purpose is to set the brakes and stop the speed increase while keeping the train stretched and preserving air for later. A full-service application at 27 mph on a 1.6% grade would produce a large retardation and could cause a run-in or wheel damage, and it would leave little margin if the grade steepens. After the application, bail off the locomotive brakes so the dynamic brake remains effective; if you leave the independent brake applied, the units can slide or the dynamics can be overridden. Confirm the DPU is applying and that brake pipe pressure is stabilizing. Then reassess speed before the 1.9% section.
2. When do you release, and what are the risks of waiting too long?
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Release before the speed falls too low and while you still have time to recharge. On a long grade, a release at high speed can let the train accelerate before the brakes fully release; a release at too low a speed can stop the train or require a heavy application to restart. Release when speed is under control and the brake pipe has had time to recharge, then let the dynamic brake hold the train. If you leave the automatic applied too long, brake shoes and wheels heat, auxiliary reservoirs deplete, and you may not have enough air for the next application. Do not release if the brake pipe has not recharged or if the DPU is not responding.
3. If speed still rises to 30 mph after the minimum reduction, what is your next step?
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Increase to a full-service application and use the dynamic brake, sand, and any retarding devices available. Confirm the DPU is not fighting the head end and that the brake pipe is not leaking. A full-service application gives more braking without the uncontrolled stop of an emergency application. If speed continues to rise toward the restriction or you lose control, make an emergency application and communicate immediately. The key is to act before the steepening grade, not after.