Euclid After reading the report on the Lac Megantic wreck, I have the same question as the original poster. The report says that when the train was secured at Nantes, the engineer left the automatic brake fully released with 94 psi in the train line (brake pipe). Only the independent brakes were set with air. It is reported that, as the train line leaked down, the loss of pressure corresponded to an equal loss of pressure in the locomotive independent brake cylinders. Eventually the independent brake cylinder pressure dropped to the point where they would not hold the train. HOWEVER: As the train line leaked down, why would not the automatic brakes apply on all the cars? They apply when there is a reduction in train line pressure. I cannot imagine that there would be so much leakage from the cylinder/reservoir side of the system that it would totally lose cylinder pressure in so short of a time.
After reading the report on the Lac Megantic wreck, I have the same question as the original poster. The report says that when the train was secured at Nantes, the engineer left the automatic brake fully released with 94 psi in the train line (brake pipe).
Only the independent brakes were set with air. It is reported that, as the train line leaked down, the loss of pressure corresponded to an equal loss of pressure in the locomotive independent brake cylinders. Eventually the independent brake cylinder pressure dropped to the point where they would not hold the train.
HOWEVER: As the train line leaked down, why would not the automatic brakes apply on all the cars? They apply when there is a reduction in train line pressure. I cannot imagine that there would be so much leakage from the cylinder/reservoir side of the system that it would totally lose cylinder pressure in so short of a time.
Because it leaked slowly enough not to set the brakes.
When the train's brakes are in full service, brake pipe pressure is maintained at 70 psi, provideng brake cylinder pressure of 50 psi. The brake pipe tends to leak at the car end hose connections. If the locomotive engine is running, a pressure maintaining feature in the locomotive brake equipent compensates for leakage. If the engine is not running, the air from the locomotive main reservoirs will eventually be depleted, resulting in gradual loss of brakes. if the unattended train had been left in Emergency, rather than Service, Brake Pipe pressure would be at zero but each car would have both the Service and Emergency reservoirs to maintain brake cylinder pressure. The locomotives could be uncoupled and the brakes would stil be effective for as long as a month. When the train restarts, it takes a little longer to release the emergency brake and return to normal condition but in the Quebec Case, 47 lives would have been saved. Lesson-Unattended trains should be left in Emergency.
leaks. The whole system is not airtight. After a while the air pressure in the airbrake cylinder in each car leaks ooff. If there is sufficient air-pressure in the cars air-tank, the air-tank will replentish the air cyliner pressure and the train-line air will replentish the air-tankikk air-pressure. On the B&M setting hand-brakes was essential for a standing train, unless the locomotive was continuing to run on idle.
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