Questions re braking

[Paul of Covington]

   I have a couple of questions about retainers.   As I first understood them, they retained some pressure on the brakes when they were released.   Later, I remember reading something about high and low settings on them to control the amount of pressure retained.   Later, there was something about gradual release, where the pressure would slowly bleed down.   Are there different types of retainers, or am I full of you know what*?

    * Misinformation

__________

(Edit):  Never mind.  I just re-read Al Krug's description.

[Overmod]

"Retarders" are specifically found on the downward slope of classification-yard humps, to control the speed of the car so it will 'just' be able to roll through the switches and bowl and couple to the standing cut with minimum impact.  Think of them as a pair of very long ground-mounted brakeshoes that bear on the rims rather than tread.  If you wondered why some trains had shiny reflective wheelrims ... they recently passed down a hump  and the contact area hasn't rusted up yet.

As I recall the 'older' position of retainers was on the side of the car, and I believe older accounts (e.g. in Railroad Magazine) describe finding them there.  If the brake equipment is on the end of the car (as it is for many tank and hopper designs), or if there is a concern with lineside obstacles or clearance problems flipping the retainer improperly, then it makes sense to put it on the end, but in doing so you lose the ability to see at a glance, as the train passes, if the retainers are up or not.

We have had several recent threads, mostly on the explicit topic of holding or controlling trains on steep grades, that have involved discussions of retainers.  Apparently many relatively new crews are not being trained in the theory and practice of the retainer system, and do not appreciate the benefit of having some of the consist given a lasting 'light set' which I believe does not go away on running release and recharge. 

As I understand it, the prohibition against reliance on dynamic for 'required' braking effort is relatively recent; in older accounts it appeared fairly common to depend on dynamics for train control on relatively steep grades well into the era of outgassing composition brakeshoes.  I wonder if it is time to reconsider the objective value of setting blocks of retainers even in this era of minimal crews on enormously overlong consists...

[BigJim]

LithoniaOperator

BTW, where are retarder levers located? I would assume they are on the bottom of cars, at the reservoirs. But I have read accounts of brakemen setting them from the overhead catwalks, in the old days; so that doesn't jive. How could one have done this from above?

You mean "Retainers"? They could be high or low on the B end of the car. I am thinking that more often than not, they were close to where the hand brake wheel was located. I have also seen them mounted to one side mid-car near where the control valve [triple-valve] is to be found.

[tree68]

LithoniaOperator

BTW, where are retarder levers located? I would assume they are on the bottom of cars, at the reservoirs. But I have read accounts of brakemen setting them from the overhead catwalks, in the old days; so that doesn't jive. How could one have done this from above?

They are now on the bottoms of the cars where they can be reached without going under the cars.

Recall that at one time brake wheels were "up" and there were roofwalks on the cars.  As such, it might be possible to set the retainers while the train was still rolling, although I don't know if that was the practice.

All you need is some form of linkage between the roof and the valve.

[Lithonia Operator]

BigJim

 

 

LithoniaOperator

There is one sentence I don't get though:"Sometimes you'll use a little power down the hill if that first service is a bit too much." Do you mean turn off the dynamics and have the engine(s) pull against the train's brakes somewhat?

 

 

Yes. The object of taking a train down the hill is to BALANCE the force of gravity against the retarding forces of dynamic braking and/or the air brakes. This fine when the grade is constant. Sometimes the grade happens to be slight enough that it can be controlled with modulating the power of the dynamic brake. Then, there are times that the grade is steep enough that the dynamic brake can no longer hold back the train. [Keep in mind that RR's limit the amount of [equivilent]axles that can be used in dynamic braking so that the buff forces do not jack-knife the train.]  In that case air must be used to help retard the train. The least amount of air, the better. This is all well and good until in the middle of this steeper grade you have to traverse a lesser grade that just happens to be so short that you don't have time enough to knock of the train [automatic] brake and have it fully charge before you start down the steeper grade again and have to reapply the automatic. In this case, I have often left the air applied and eased off the dynamic brake and pulled the train through the flatter section until the dynamic was needed again.   

 

 

 

LithoniaOperator

If so, is that bad for the brake shoes/wheels if done too long?

 

 

On normal grades, this would not be a factor. However, there are grades in places around the country that are very steep. So much so that speed down the hill is limited by timetable rules so that the brakes shoes do not get so hot that they lose their effectiveness. This has been the cause of a number of derailments where the train has run away because the speed was allowed to get over the point of braking effectiveness.

 

 

 

tree68

Cycle braking can be what gets a train in trouble, if there is insufficient time allowed to recharge the reservoirs on the cars.  Each application provides a little less braking force, until finally there is none at all.  A good many runaways have been attributed to exactly this phenomenon.

 

 

Let me say this about that:
tree68 is correct, but, there is a bit "relativity" that needs to addressed. When you first apply the automatic brake from a FULLY charged trainline, let's say a ten pound reduction, there is a certain, "comfortable", feel to the train when the brakes set up. Each time you make this reduction from a fully charged trainline, you get that same feeling from the train.
Now, if you release the brake and have to very shortly make another application, you don't get the same feeling from that ten pound reduction. Why? Because the braking system isn't fully charged, and, because it isn't fully charged, the brake control valve doesn't apply as much pressure as it would have if it had been fully charged. So, you have to make a little further reduction in order to get back to that "happy" place you were in to begin with where the contol valve applied the same amount of pressure as before. This happens every time you "fan" the brake, until you get to the point where you have what we called "pissed all of your air away"! 

That was all very interesting and helpful, BigJim. Thanks so much. I think I get the picture now.

I've read articles about the big CSX Sand Patch derailment. Some say the engineer did not brake frequently enough to keep snow/ice from building up between the shoes and the wheels. Others say he cycled the brakes too much and, and, as you say, pisssed away his air. Maybe this is simply a difference of opinion which endures. ??

In any event, being shoved down that grade by 100+ loaded coal hoppers, at speeds reaching the posted limit plus 40 mph, must have been truly terrifying.

BTW, where are retarder levers located? I would assume they are on the bottom of cars, at the reservoirs. But I have read accounts of brakemen setting them from the overhead catwalks, in the old days; so that doesn't jive. How could one have done this from above?

[BigJim]

LithoniaOperator

There is one sentence I don't get though:"Sometimes you'll use a little power down the hill if that first service is a bit too much." Do you mean turn off the dynamics and have the engine(s) pull against the train's brakes somewhat?

Yes. The object of taking a train down the hill is to BALANCE the force of gravity against the retarding forces of dynamic braking and/or the air brakes. This fine when the grade is constant. Sometimes the grade happens to be slight enough that it can be controlled with modulating the power of the dynamic brake. Then, there are times that the grade is steep enough that the dynamic brake can no longer hold back the train. [Keep in mind that RR's limit the amount of [equivilent]axles that can be used in dynamic braking so that the buff forces do not jack-knife the train.]  In that case air must be used to help retard the train. The least amount of air, the better. This is all well and good until in the middle of this steeper grade you have to traverse a lesser grade that just happens to be so short that you don't have time enough to knock of the train [automatic] brake and have it fully charge before you start down the steeper grade again and have to reapply the automatic. In this case, I have often left the air applied and eased off the dynamic brake and pulled the train through the flatter section until the dynamic was needed again.   

LithoniaOperator

If so, is that bad for the brake shoes/wheels if done too long?

On normal grades, this would not be a factor. However, there are grades in places around the country that are very steep. So much so that speed down the hill is limited by timetable rules so that the brakes shoes do not get so hot that they lose their effectiveness. This has been the cause of a number of derailments where the train has run away because the speed was allowed to get over the point of braking effectiveness.

tree68

Cycle braking can be what gets a train in trouble, if there is insufficient time allowed to recharge the reservoirs on the cars.  Each application provides a little less braking force, until finally there is none at all.  A good many runaways have been attributed to exactly this phenomenon.

Let me say this about that:
tree68 is correct, but, there is a bit "relativity" that needs to addressed. When you first apply the automatic brake from a FULLY charged trainline, let's say a ten pound reduction, there is a certain, "comfortable", feel to the train when the brakes set up. Each time you make this reduction from a fully charged trainline, you get that same feeling from the train.
Now, if you release the brake and have to very shortly make another application, you don't get the same feeling from that ten pound reduction. Why? Because the braking system isn't fully charged, and, because it isn't fully charged, the brake control valve doesn't apply as much pressure as it would have if it had been fully charged. So, you have to make a little further reduction in order to get back to that "happy" place you were in to begin with where the contol valve applied the same amount of pressure as before. This happens every time you "fan" the brake, until you get to the point where you have what we called "pissed all of your air away"! 

[Lithonia Operator]

Tree, thanks so much for your excellent explanation. I really appreciate your taking the time to write all that.

There is one sentence I don't get though:"Sometimes you'll use a little power down the hill if that first service is a bit too much." Do you mean turn off the dynamics and have the engine(s) pull against the train's brakes somewhat? If so, is that bad for the brake shoes/wheels if done too long?

Oh, and by "cycle braking" do you mean on/off/on/off as I had described?

I will definitely read the linked article.

Thanks again!

[tree68]

LithoniaOperator

My understanding is that RR air brakes are either fully on or fully off.

Not so.  The train brakes can be applied in increasing increments.  A "first service" is about a six pound reduction of the pressure in the brake pipe.  "Full service" is the point where you're braking as much as possible, that's about a 26 pound reduction.  Once you've made a first service, you can continue to reduce the pressure in the brake pipe (ie, step harder on the brakes) until you reach full service.

It's the release that's all or nothing, at least on freight trains (passenger has a feature called "graduated release).  So you can't just let off on the brakes a little bit if you find you've taken too much - you have to fully release the brakes, which must then recharge.

In come cases, a first service application may do a nice job of holding the train on a downgrade.  If it does, you're all set.  If it doesn't, you can add a little more if needed, or you have to release the brakes entirely.  Sometimes you'll use a little power down the hill if that first service is a bit too much.

If you are using retainers, some of the pressure in the brake cylinders on the cars will be "retained," effectively leaving a set on the cars.  Meantime, the engineer can release the brakes and recharge the system.

If dynamics can't hold the train (and I've slipped the dynamics on wet/leafy/icy rail), then air will be necessary.  

Cycle braking can be what gets a train in trouble, if there is insufficient time allowed to recharge the reservoirs on the cars.  Each application provides a little less braking force, until finally there is none at all.  A good many runaways have been attributed to exactly this phenomenon.

Time to recharge will vary, but will definitely be much longer for longer trains, in cold weather, etc.  On my short trains, it's measured in seconds, but a long train might take a few minutes, especially from a deep application.

If you haven't already read Al Krug's excellent primer on railroad air brakes, you'll find it well worth your while. http://www.railway-technical.com/trains/rolling-stock-index-l/train-equipment/brakes/north-american-freight.html

[daveklepper]

You are mistaken about airbrakes being fully on or fully off.   Both airbrakes znd dyazmic brakes are capable of gradations.  And for airbrakes this applies to both the train-line airbrake, which acts to reduce pressure in the train line, with "dumping" or "big-holing" meaning exhausting the train line as rapidly as possible being the emergency application, as well as the independent locomoive brake, which simply feeds pressure to the locomotive's brake cylinders.  The right amount of train-line pressure reduction for a partiular situation is one of the important skills of a good engineers.   With dynamics, the right comibnation is important.   There are times when cycling airbrakes between off and some pressure reduction may be useful, but I will an explaition of why and when to others running trains regularly.

[Lithonia Operator]

I have a basic understanding of how air braking and dynamic braking work on a train. But in discussions I’ve read about retainers, there is mention that dynamic brakes have almost rendered retainers obsolete. Is it actually meant that the combination of normal air braking and dynamics means retainers are not necessary?

Say a heavy 120-car train is descending a 1.5% grade. I assume the dynamics would be hard at work continuously. But what else is going on? Is the engineer doing a service application of the train-line air brakes at regular intervals? My understanding is that RR air brakes are either fully on or fully off. So would our hogger, say, do a 1-minute application every 5 minutes, or something like that? How long does it take to deplete the individual reservoirs on the cars? Then how long does it take to re-pressurize the train-line (and reservoirs)? Even six big units with dynamics would seem to be little match for 120 loaded hoppers pushing from behind on a steep downgrade. Also, without airbrakes, it would seem like the train would try to overrun the locos and jackknife, even if the dynamics were that strong. So what exactly is the routine? I’d love to hear from an experienced engineer. Sorry if my terminology is incorrect.