When you set the 'automatic' the train brakes will start setting from the point of application and approximately one car at a time. This is why you want the train either fully bunched or full stretched when you go to set the brakes for good train handling. I say 'point of application' because when running with DP, and no 'fence up', the rear DP unit will make a matching application of the brakes and will also set the brakes from the rear. That is why trains with DP will stop and 'go' faster, you're setting the brakes from both ends, and charging the train from both ends.
There is something called a 'bake pipe gradient' through the train. The locomotives are putting out 90 lbs of air, but by the rear of the train it has been reduced. Leaks, brake pipe length, etc play into this 'gradient'. Brakes set up because of a 'difference in pressure' between the brake pipe and the service portion of the brake reservoir. The engineer makes a reduction in the brake pipe, the triple valve responds by taking air out of the service portion and putting it into the brake cylinders. On a 'conventional' train the 'gradient' is a slope. On a DP train, it is a shallow 'v'.
On another thread they talked about 'pissing away the air'. To set the train brakes, air is taken from the 'service' portion of the brake reservoir of the car, and put into the brake cylinder, forcing the piston out. For this example, you take 10 lbs. Once you've slowed the train, you knock the air off. Air starts flowing to the brake pipe again, releasing the air out of the pistons, and starts filling the 'service' portion of the reservoir again. If you don't have time to fully charge the brake pipe, and you need to make another reduction, you have to take 3 more lbs of air then you did on the previous reduction.
If you take 10 lbs again, you'll probably get a few brakes to set up on the train, but you're not getting a uniform brake application through the train. A good engineer will note the brake pipe pressure on the HTD of the rear to get a general idea of when his trainline is 'fully charged'. Most of the class I's have "Air Flow Meters" which measures the amount of air going to the brake pipe. You need less than 60 cfm to be 'legal' in the FRA's eyes, but when fully charged its wise to see how much air is flowing into the pipe from the locomotive, again to know when the brake pipe is fully charged.
That is why when slowing the train, if you're immediately reaching for 20lbs of air or full service, you're bordering on out of control. Trust me, I've been there. Coming down Bellefountaine with a 14,000 ton train, right at 50, and getting an 'approach' for a train stopped at Ridgeway. I was already in full dynamic and first service to keep it at 50. Went full service and even used a liberal application of the "Flintstone Brake". I was complying with 'restricted speed' at 127 at the bottom as far as not 'speeding', but thankfully it was short train ahead. A little heads up from Indy to the fact a train was stopped at Ridgeway would have gone a long way there...
Just to overwhelm you, the cars also have an 'emergency' portion of the reservoir in addition to the 'service' portion. The 'emergency' portion is what keeps the brakes applied on a car when the car is in 'emergency' and no air is flowing to the system. Once the 'emergency' portion is drained (either by a leak or using the bleed rod) the brakes on the car will release. This is why railroads have rules about not relying on the 'air brakes' to hold a cut of cars, and why handbrakes must be applied. Eventually the brakes are going to release, and those cars will be 'free rolling'. This is what is confusing to a lot of 'young' railroaders. You need air to release the brakes, you need air pressure to set the brakes but no air in a car means the brakes are released. That Westinghouse guy was one smart cookie.
Mr. Tops wrote:
The reason you bail off the independent brake to release the engine brakes (except when coming to a stop) is to prevent sliding wheels, which would create flat spots; and reduce the chances of excessive slack run-in, which could result in a derailment/track damage if severe enough. It also puts less wear and tear on the locomotive brake shoes, so they don't have to be replaced as often.
"Bailing off" is more of a train handling thing, though wear and tear on the brakes does come into play. You're not going to flat spot the wheels with only 10 or 15 lbs of air set. Again, if you're having to use full service to stop for a signal or something (other than non-emergency) then you're bordering on 'out of control'. I liked using dynamics and about 10 lbs of air. You could then put a little more air on to stop quicker or use the dynamic brakes to slow. Plus at 10 lbs reduction, you could kick the brakes off without fear of sticking brakes because they didn't release. "First Service" is nice for conditioning brakes for a stop (getting ice and now off the rigging) or for starting to slow the train if you know you're going to bring it to a full stop. Once stopped, set the res equate 20 lbs of air and take a break.
However, dynamic brakes cut out once there is 10 lbs of air in the locomotive brake cylinders. You don't want to be in full dynamic, set 10 lbs of air to bring the train to a stop, and have your dynamics cut out. That's why you 'bail off' when in dynamic. When stretch braking, you 'bail off' so that the brakes aren't 'slowing' the engine while you're trying to keep it fully stretched out.
Using the automatic brake will give you a little additional pressure in the locomotive brake cylinders when you go to full service. This is helpful in yard switching service where you're not using air through the entire cut. Gives you a couple extra pounds of air to stop with, provided you don't actuate the air off out of habit. Also, the locomotive brakes can be set up so that using the automatic will not 'slide' the brakes. Conrail's SD38 and MT6 (and conceivably most of their yard service stuff) was set up such that it was nearly impossible to slide the brakes while using the independent or automatic brakes. I've actually run yard power around on a very snowy and cold morning with 10 lbs of 'automatic' set. This let me start to warm the brakes up in preparation for switching. This was the same morning where even throwing the engine in emergency wouldn't stop it. Thankfully the track leveled out and we could finally get stopped. This is something that will catch a lot of engineers off guard, especially 'new' ones. When the brakes are cold, they don't stop for anything. In my case, I was expecting the engine to have difficulty stopping, just wasn't expecting to not stop at all...
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