From time to time I read about situations where empty cars are added to a train for extra brakeing. Wouldn’t the weight of the extra cars cancel out the added brakeing effort?
not really – since the brakes apply with (hopefully!) equal force on each wheel, then each wheel will have equal retarding force, and the empties will help to stop the loads. Unless the empties develop wheel slip…
Hey, One more post and you are a four star general Jamie, Congrats.[8D]
Doesn’t the load sensor limit the brakeing on the empties?
It seems to me like it would be the same as you trying to slow your car with your tennis shoes.
The railroads use Tons per Operative Brake. A loaded grain car will have 143 tons per operative brake. Add one empty grain car that weighs 25 tons (aluminum car for example) and you now have two cars with working brakes which total 168 tons. That works out to 83 tons per operative brake or TOB. So the two car train will tend to stop quicker than the one loaded car train will. Two loaded grain cars will give a TOB of 143 and in fact 110 loaded grain cars will have the same TOB as a single loaded car, 143 TOB.
Other factors like load/empty sensors, brake shoe type, age of the equipment can be factored it but mostly it is just how many cars are trying to hold back how many tons.
OK, but the loaded car has 143 tons worth of adheasion and the empty only has 25 tons. So the empty couldn’t come anywhere near the adheasion of the loaded car. Empty / load sensor aside, wouldn’t the empty just get draged along (wheels slideing)? And with the load sensor the pressure to the shoes, wouldn’t they de-rate enough to be useless as far as holding back the load?
Thanks Mark,
Now that I think about it the stories I read where this was practiced are all from the old days. It makes a little more sense now.
It will suprise you how much braking effort a few cars will give you.When we switch in the yard we go down hill aliitle bit and at times have a cut of 50 or more cars(without the air being worked on all the cars).So the conductor will snap the hoses between a few cars and presto your braking power is greatly increased.Plus it saves those shoes on the engine,and helps prevent flatspots.And if your talking about empties being added to a big main line train,well I 've seen it happenon a mixed train (coal loads with empties on the rear) go in emergency and because the empties were on the rear and their brakes set and apply quicker than the loads,bust a knuckle on the load coupled to the the first empty.
mackb4,
Why do they apply quicker? Is it because they don’t fully apply because of the load sensor?
Load/empty sensing equipment never caught on, that I’m aware of.
Car designers refer to braking in terms of braking ration. That is the braking force divided by the car’s wt. The trick is keep the braking ratio for the car when loaded high enough so that trains can run at reasonable speed and still get stopped in a block length but not so high that the wheels slide when empty.
This balancing act gets really hairy with hopper cars. The goal is to keep the tare wt. down so you can load more coal, but that leaves you with a really lousy loaded braking ratio - something like 8%.
This is the real reason trains take so long to stop at speed. It’s not their mass, it’s the low braking ratio! Theoretically, with load/empty sensing equipment, you could design for 20% or so braking ratio which would drastically reduce stopping distances. Add in wheel slide sensing, and you could get up to 30% or so. ECP would make all this practical, but the $$ and will to do it aren’t there, yet.
There was a photo in “Railroad News Photos” in TRAINS many years ago with a heavy load on a depressed center flat car being lifted up Madison Hill by both PC SD7’s (double headed due to no MU) and about 12 empty cars added for additional braking effort.
Something interesting a roadforeman told me once.He said a study was made that showed a solid bulk commodity train,such as coal or grain,actually picked up speed for a slight instance when the air brakes were applied in emergancy(that is being shot or dumped).Must of been that the train slid at that instant of increase of speed ?Coming in off the Valley side into Portsmouth Ohio,the time table at one time recommended stretching the train into the yard account of the big dip entering.I don’t know if it’s still in there ?
When ever we have a good sized cut of loads in our switching cut, I always lace up the head few cars…you would be amazed how much of an effect a few cars of air can have when flat switching…lets the engineer really crack the whip on 'em, which makes my job a lot easier.
I really hate running with the pin…
Ed
The factors of adhesion do not seem to be as critical as you suppose. If the rail is dry or not greased by curve oilers the brakes apply and the wheels continue to roll. There are instances where I have seen wheels on empty cars sliding along the rails on heavy trains in mountain territory. I think this has more to do with improper air brake proceedures than the physics of the cars and adhesion.
Load/empty sensors are now the norm on cars used in unit train service as well as intermodal equipment. You just get it figured out how to take an mty grain train down the mountain where a minimum application is too much air and there is not enough dynamic braking and SURPRISE you get a grain mty with all new equipment. You go through your preferred routine and all of a sudden you are going way to fast. The FRA regs now require you dump the air on a train in mountain grade territory as soon as it reaches a speed which is 5 mph above the posted speed which is a really stupid idea from my experience but you either comply or just grab some more air. The common figure for a train in mountain grade territory is 1# of brake pipe reduction for every 10 T/OB. That is a pretty good guideline though auto racks will take more as will stack cars. All of a sudden your train with 34 T/OB requires just as much brake pipe reduction as a loaded grain train with 143 T/OB would in the same territory. Yes the load/empty sensors work really well, at least when they are new. No, you cannot set less than about 7# of air account the way the brake valves on the cars work. That would be another lesson you teach your trainees with the preface, “Now this is how you do it if you have to but the Roadforeman will not be happy if he sees you doing it on your tapes. He did it the same way when he was out here running trains but now he has to toe the company line.”
I don’t think I have ever heard of a railroad adding mty cars to any regular train for increased braking capacity. You see it on special moves for extrem
emptys slow down faster then loads for the simple reason of mass… it takes the loads longer for the breaks to heat up and begin to slow the car down becoues it has more mass to slow down…as its the opposit for an empty…its physic 101…
csx engineer
Also consider weather conditions haveing snow and ice bulid up on shoes makes alot of differance in breaking, rain also.
Rodney
P.S. I failed physics in high school and I am running a train. (lol)
Thanks for the answers guys.
If memory serves (it doesn’t, always…) at least some ore hoppers – iron ore and other heavy stuff – had two different brake systems, one for empty and one for loaded.
And thanks for the congrats, Chad!