Since the dynamic brake uses the motors that move the engine as generators, the locomotive must be moving for the dynamic brake to work. However, at very low speeds, the now generators do not provide enough drag to serve as brakes, so air must be used to completely stop the train and hold it in place.
Johnny
Some locomotives have the traction motor blowers (cooling fans) attached directly to the prime mover, so the prime mover actually revs up when going into dynamics (I think the SD40-2 is an example).
GE has marketed a hybrid where the prime mover shuts down on hills, but no orders have been built.
The SP San Bernardino derailment at Duffy Street in 1989? That was caused more by the miscalculations of weight that caused the train to be overloaded.
Dynamic brakes were automatically canceled with an emergency brake application. This is based on a now reversed FRA mandate, of which I'm not sure why it was put in place. The dynamics could not hold the train, but could have better held it than the air brakes. Also contributing was the inoperable dynamics, perhaps related to SP's shaky financial status at this time.
On most diesel-electrics, the prime mover still has to be working, at least at idle speed, possibly greater depending on the blower load. The auxiliaries need power, the blowers, and even if much of the control and accessorry equipment runs off battery power, you do not want to drain the batteries on a long downhill grade. A hybred with larger batteries is a different mater.
they should known that they were coming into that zone any way so even if they didn't they should have at least slowed down.
NorthWest The SP San Bernardino derailment at Duffy Street in 1989? That was caused more by the miscalculations of weight that caused the train to be overloaded. Dynamic brakes were automatically canceled with an emergency brake application. This is based on a now reversed FRA mandate, of which I'm not sure why it was put in place. The dynamics could not hold the train, but could have better held it than the air brakes. Also contributing was the inoperable dynamics, perhaps related to SP's shaky financial status at this time.
There were several theories for knocking off the Dynamic brakes with an Undesired Emergency brake application. The first was that the additional brake pressure to the locomotive wheels would lock the engine wheels - thus diminishing braking effort. Of course the Engineer could 'bail off' the locomotive air brake application. The second theory was that when a UDE occurred you wanted the head end of the train to continue running ahead of the rear of the train, presuming that the train had something that caused the train to be in two pieces, so that the rear of the train would not collide with the stopped head part of the train.
It is vitally important for the engineer to know how much dynamic braking his engine consist actually has working. Most idiots can get a train to climb a grade - an Engineer know how to get the train down the grade safely.
UMMV!
Never too old to have a happy childhood!