So, I have seen multiple times on multiple threads on this forum alone, from experienced former (and some active) railroad workers who say that putting a train into emergency is not a good idea, which makes sense. However, would slowly applying the brakes (like reduction, minimum application, suppression, full service, then emergency) work because it is not a sudden drop in air pressure, so it is not as much of a sudden application?
The carriers are doing their best not to introduce air brakes into their braking regulations for their trains. A significant percentage of routine braking events are being done with Dynamic Braking with todays AC traction locomotives with extended range dynamic brakes.
Air brakes when used, wear away brake shoes and apply heat into wheels - and driving up maintenance - brake shoe replacements and wheel replacements as the brake shoes are designed to wear away and occasionally axles will lock under braking and slide the wheels to the point of condemnable flat spots. A secondary problem in the use of air brakes is the ākickerā or ādynamiterā - the car that has a defective brake valve that reacts to a service brake application by initiating a unintended emergency brake application.
I am certain we have seen photographs of trains descending grades with most of the train behind the engines being nearly hidden in swirling brake smoke being generated by the brakes applied on the cars - in many cases with Retainer Valves being manually set to retain the brake application to some extent, even when the Engineer has commanded a brake release by the operation of the engineās brake valve. For the most part we no longer see this in the 21st Century due to Dynamic Braking handling the bulk of braking and air brakes being used to supplement the power of Dynamic braking.
The brake reservoirs on cars are bifurcated - one part of the reservoir is used by service braking, the other part (bigger volume) is used for Emergency braking. When the emergency system is activated it applies the car brakes with more power than does the service braking. Emergency, once initiated is uncontrolled. You donāt want anything to happen āuncontrolledā if it can be done with a controlled action.
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Ohhhh, ok
āEmergencyā is intended to produce what the Navy calls a crash stop, both applying in minimum time and stopping with maximum force. There is no way to graduate the application, or release it before you come to a locked-wheel stop.
Big-holing at any point in a modulated application will result very promptly in some appreciable fraction of emergency reservoir pressure being applied to brake cylinders that will already have appreciable pressure in them, and with substantially all the slack out of foundation gear or shoe contact. The result will be a very sharp, irrevocable brake application, probably sliding wheels even at comparatively low speed, and requiring the usual walking-the-train-twice procedure afterward.
The only problem with extending a service release through multiple sets is that the reservoirs wonāt start to recharge (on ABDX) until the brakes have been fully released. So by the time you actually get to full service the available pressure you can develop in the brake cylinder may be limited to the pressure you have left, which may be considerably lower than that in the emergency reservoirs (and the trainline).
A big advantage of ECP is that the trainline and the service reservoirs can run in equilibrium all the time, so you can have high pressure available even during long progressively-increasing sets. Another advantage is that you can have graduated release without having to fully release and then reset.
The simplest answer as to why engineers donāt make a service application before an emergency application is time.
A service application travels at about 540 feet per second. Letās just say about a mile in 10 seconds. A two mile long train is going to take 20 plus seconds for all the brakes to set up and provide some braking effort. Ideally, that would help control slack action.
However, emergency is called that because itās reserved for those situations where itās been determined you need to apply the brakes fast. You donāt have the luxury of time to get some braking action going before going to the ābig hole.ā
That being said, Iāve had some situations that were developing where an emergency application might have become necessary and I made a minimum reduction. Emergency wasnāt needed and I made sure the brake pipe reduction was at least 10 psi and then released the brakes.
Iāve also had a couple situations where emergency was immediately needed, no time to get service braking effort out of the train.
Emergency applications initiated from the train, a dynamiter, knuckle breaking or an air hose coming apart, etc. are a completely different animal. Those kinds of emergency applications are the ones engineers will deal with the most in their career.
Jeff
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Is the use of dynamic brakes over air brakes SOP for most, if not all railroads? I live right next to, in fact my driveway crosses over one of the UP main lines. Trains are stopped and blocking my driveway for usually a short time regularly, sometimes daily. Sometimes itās to let a higher priority train pass, sometimes itās because crews run out of hours( my assumption), and they chauffeur another crew in to swap them out, and occasionally because of some kind of mechanical issue.
I can always tell when they are getting ready to move again because 1) the horn signal, and 2) you can hear and see the brakes release.
So, and once again this is only an assumption since I have no actual knowledge or experience on the subject, with what the knowledgeable people have said, Iām guessing that since 99% of the time itās a planned stop, and being the Midwest and virtually flat, they are using the dynamic brakes to slow and stop the train, and only applying enough air once stopped to act as a āparking brakeā.
Would this be a correct assumption? And BTW, thank you all that are knowledgeable for sharing your knowledge with the rest of us.
Also, my apologies for the long post and extra- long sentences within.
For maintaining speed or slowing down, throttle modulation is the preferred method. When that alone canāt control speed, then dynamics supplemented by use of air brakes is the next method. Using air brakes alone is the least preferred method, but there are times when air brakes are the best method for controlling slack in the train.
Because there are times when power or stretch braking is needed, itās tolerated now more than it was 10 or 15 years ago. For me, releasing the air brakes while above throttle notch 2 was considered power braking. A few years ago it was changed to being above notch 5 when releasing.
When stopping, air brakes will be used for the final stop. Itās to control slack and hold the train. When ready to start again, the release of the air brakes and subsequent rise of brake pipe pressure at the end of the train shows you have brake pipe continuity through the train. That no one turned an anglecock, closing off the flow of air.
Jeff
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