Western Railroad Discussion > Stopping Distance equation?

A few months ago someone here had a "seat of pants" stopping distance
formula --
(It was something like; total length divided by speed)

Does anyone remember that string or have that 'equation'?

Thanks

Tim G over

I am not sure what kind of stopping distance you are looking for, but there are several kinds: power braking, dynamic braking, and emergency braking to name a few. The only formula I have ever seen applied concerns emergency braking. As a rule of thumb (this was taught to us in engineer training school back in 1973), if you are traveling between 10 and 70 mph, if you square the speed, that is roughly equal to the distance a mixed freight will travel in feet from the time of the emergency application to an on-the-rail stop (jackknife those babies and they will pile up in a relatively short distance). Having experienced many undesired emergency brake applications due to dynamiters in the train, I can tell you that on flat track that works pretty well. Add some hills in there, one way or another, and that formula goes to pot. Also, empty trains will sit down on you in about half that distance, and loads will shove you forever (or so it seems).

And, as Engineer Mr. T would say, I pity the fool with empties ahead and loads behind.

So, using that formula, a train traveling 60 would go about 3600 feet before sliding to a rough stop in an emergency brake application. And, if the engineer forgets to hold some pressure on the independent brake, the engine will go just a little bit further than the rest of the train (also called knuckle city- just like popping a wet towel).

As far as using an equation for routine stops, I developed my own based on my style of braking, but it would not work for anyone else unless I trained them and they followed my "formula for remaining employed." All engineers on my road were power brakers, and we could stop a train in a much shorter distance than a dynamic braker because we used the train brakes from the get go.

There are too many missing factors:

Speed, Grade, Tonnage, Equipment, just to name a few.

Nudge

Nudge, of course each train handles differently. I already said that hills nullify the formula. But, unless you have a unit train (loaded or empty), the formula works fairly well- good enough to satisfy many a Santa Fe law suit where there was a question of speed. Try it the next time you are rolling along and an air hose bursts or one of those cushion drawbar cars stretches out too far and breaks the gladhands apart- start counting those poles. You will be surprised at how close the speed squared formula works. The UP computer simulator comes closer, but you factor in each car, the weight, the weight distribution, the grade, curves, etc. But, as a rule of thumb, speed squared does the trick. A friend of mine used it to decide whether to jump from the engine or not when the train he was passing couldn't get stopped in the siding and ran out in front of him. He had passed the east end signal and was running 50 mph next to a 5,000 foot siding when he saw the engine turning out to the main at the west end of the siding. He plugged it, did a quick mental calculation, and decided he would stop in about 2500 feet, with about 1,000 feet to spare. The head brakeman bailed off while the train was still moving about 40, landed on a pile of crossties, and was pretty boogered up. I imagine the pucker factor was high as my partner wondered whether that speed squared was good enough, and it was.

Try it- you'll like it!

Gee whiz Bob. In this day of kid's who can't make change for a dollar (whthout a computer) much less square a number in their head, does anyone even teach that stuff any more?

Mike Harris aka RailNet 802, out

Mike, it's been 12 years since I had a student engineer, but they were picking stuff up pretty well back then. We were making engineers out of 18-year-olds. Hey- it worked for the Air Force- some of the best aces were still in their teens when they shot down that 5th fighter. My son has been flying jets since he was a junior in high school. But, you still get some duds.

I was assigned a trainee back in the 70s, and I spent a couple of hours diagraming the brake system for him, explaining this that and the other thing. When I paused to see if any of it had sunk in, he said, "Guess what?" I said what. He said "I made a hole in one today." Needless to say, he sat in the fireman's seat for quite a while before I would let him sit behind the throttle. He finally made a decent engineer, and is still working in Baytown the last I heard. You ever hear an engineer Savoy on the UP radio coming in to Settegast from Baytown, that's my boy with the hole in one. After him, I would ask the trainee if he was a golfer before explaining anything.

Bob

I figured I'd get some info fast from you guys-
Thank you-
PB- That's the one I was remembering-
Unfortunatly, I'm not real fast with math, so...

Here's the scenario: (it's a 6.27 thing)

SD45 w/ 50 loads, 500ft visability on a curve.

In this case I think Nudge has it right, not enough info to determine
a safe speed.
Grade and other unstated variables make determining an official 6.27
speed almost impossible. My gut tells me it's about 7mph, but that would be a delay, right?

Still scratchin' my head,
Tim G

...that additional braking effort can be obtained by firmly pushing the "auxiliary brake" with one's foot. Doing so allows the train to stop within 18 inches of a red signal.

And if one grits his teeth as tightly as he can while simultaneously pushing the "auxiliary brake", he can get it down to 12 inches or less.

(The "auxiliary brake" is the engineer's cab heater housing.)

Tim, A "delay" is anytime you are stopped for 10 minutes or longer, or at least it was in my days. Stopping for a red block is not a delay, and stopping for less than 10 minutes is not a delay, and running slower than maximum speed is not a delay.

With 500 feet visibility, you have to be able to stop in less than 250 feet. That's 5 car lengths. If you want to make a service stop rather than an emergency stop, 5 mph is more like it, and that may be too fast depending on the train. I would hate to have to try to stop a 100-car train in 5 car lengths- it can take you that long to gather the slack! Slap some air under that baby and pull that train around the curve with the brakes set- that's the powerbraking way. Once you have a 6-pound reduction under a train and you are pulling on it, you can hit it with another 5 pounds at 10 mph and stop in 100 feet, no emergency application needed. I sure wouldn't hit a blind curve with no air under the train.

Before the UP merger, we worked under the Uniform Code of Operating Rules. Under the UCOR, all you had to do was be able to stop short of a train, engine, obstruction, or switch not properly lined, looking out for broken rail, and not exceeding 20 mph. That was "Low Speed." Restricted Speed was "Proceed prepared to stop short of train, engine, obstruction, or switch not properly lined." Note we could run maximum speed if we could see far enough. Restricted speed had nothing to do with block signals- it was used on tracks other than the main, such as industrial leads, and allowed you to typically run 30 mph or faster, and it was the speed required in yard limits if you entered the limits on other than a clear signal. Obviously, although the rule didn't state it, you had to consider that someone could be moving at the same speed in your direction. However, most people didn't think of it that way, and several head-on collisions resulted. The MP actually came out with a revised Low Speed that threw in the "a speed that will permit stopping within half the distance" clause, and that carried over into the GCOR.

On the little tourist line I now work for, our entire track is restricted speed, so we are always creeping around curves. We used to have block register territory, and once the ribbon rail is finished being laid, we hope to have those rules in place again so we can cruise on around those curves.

Bob C

Count poles? What are "poles"?

Set air and pull them into the restricted curve? You are fired buddy. No powerbraking allowed. The automatic D/L of the event recorder and the computer search for powerbraking violations will squeal on you and send a report to your RFE and Supt. Doesn't that steam engine you run do that? ;-)

I hit a pickup truck with an intermodal at 58 mph. I dumped it at moment of impact. The train was 3200 feet long. The last car was about 100 ft clear of the crossing when we stopped. It was warm and dry and essentially level track. 58x58 = 3364 feet. Bingo.

I'm sure Bob knows this but for the rest of you....
I have measured this time after time using my watch and Fred and the footage counter. If you set a minimum to 8 psi on an 80-110 car train it takes approx 8-9 seconds for the air pressure on the FRED to start dropping. At 40 mph you have traveled 500-600 ft. It takes another 20-30 seconds for the brakes to get seated and for cylinder pressure to build and for the brakes to really begin dragging. You have gone a total of 2000 ft or more. If the brakes are wet or snowy you can about double that time and distance. How much longer it takes to stop depends upon grade, TOB, etc. At Sheridan depot I set the minimum at 40 mph about 6,000 ft out and in number 3 or 4 throttle. As the air takes hold I reduce to 2 or 3. (power braking above #4 is illegal). It is uphill about 0.5% but a short steep grade of about 0.8% 2500 ft from depot, then the headend tops over a slight downgrade a few hundreds feet before the depot. Usually all this brings you to a halt at the depot door (6,000 ft). But with heavy trains you need to increase the set to 10-11 psi 600-1000 ft before the stop. With grain empties or coal empties I start about 4500-5000 ft from the depot instead of 6000-6500 ft.

It doesn't alway work like that. Trip before last trip it was 50 degrees, dry. I had a 102 car earthworm grain empty with 3 C44s but only two on line. From previous experience with this train over the road I knew I'd better start early. I set 7 psi 8500 feet from the depot at 45 mph and was in number 3 throttle. I reduced to number 2 about 4500 ft from the depot. 1500 ft from the depot I had to increase the set to 12 psi to stop at the door. Geeze I hope they work better than that when loaded!! The load/empty devices on some of these trains goes a bit overboard if you ask my humble opinion. I warned the relief engineer about it.

Coming home last night on an H train (freight) I had 3 SD40-2s and 84 cars, 7005 tons. It was minus 14 degrees F and the train was blowing snow up from the right of way. I set 8 psi before topping over 1.25% Parkman Hill at 12 mph. As the train picked up speed topping over I went to idle then bunched them in DB. (Air brakes still set). By 38 mph I was in full DB. The speed leveled at 45 mph (legal limit below zero degrees) on the little flat spot at MP 721. Then it began to increase as more cars came off the flat. I had to increase it to 12 psi at MP 720. At MP 718 I had to increase it to 18 psi as we were now doing 46 mph. The needle kept climbing so I put 18 psi in her at MP717. It reached 18 mph then finally she started to slow, barely. The grade lessens considerably by there but it rolled on at 45 mph. Fun and games. Normally a 7-8 psi set with full DB is too much for these trains down the hill. You have to cycle the air on and off 2 or 3 times. But this train was one long 18 psi set. My thoughts are: 1) The blowing snow lubricates the brakes. 2) The air was leaking out of the -14 degree brake cylinders about as fast as I was putting it in. So you can't depend upon a rule of thumb. Each train is different. You need to know what it SHOULD be doing at this point and make adjustments to compensate.

AK

I should proof read better.

The set at MP 718 should have read 15 psi not 18 psi. The speed reached 48 mph not 18 mph.

AK

Al, you say "illegal", and that is misleading. It may be a BNSF rule, but a lot of railroads still allow powerbraking. It is required on mine because we do not have dynamics. I don't advocate using it if the rules say no- I do believe in living up to the rules. So, If I were on the BNSF I would be using blended braking instead of powerbraking. There is no one on the caboose, so not much reason to worry about slight slack adjustments.

Our rules even require powerbraking during switching operations. Of course, we are switching occupied passenger cars when we switch!

I have worked #8 throttle all the way to within 10 feet of a stop with 6000 hp. You have to be able to sense that wheelslip and get off the throttle or risk a knuckle. I never got one doing this. The rules I worked under required a final reduction just before stopping, allowing the independent to set, and placing the throttle in idle just before coming to a stop.

On the AB equipment, it took 30 seconds for the brakes to begin to apply after making an initial reduction on a 125-car (125 cars to the mile back then) train. That meant the first half mile was used to bring the brakeshoes into contact with the wheels. If we were running 60 mph, at the end of the first mile after making a 6-pound and then a 2-pound reduction, we were still running 55 mph. Follow-up reductions would bring us to a stop in the next mile. If I was running 50, I started my initial reduction at 1.5 miles from a stop. When the ABD brakes were introduced, you could start 1/4 mile later because it only took 10-12 seconds for the reduction to work its way back to the caboose. ABDW brakes helped even more. You could powerbrake from 50 mph to a stop in 1 mile with a 125-car (now it was more like 1.75 miles long) train.

Dynamiters? We did probably the same thing as someone who uses blended brakes- we used the feed valve (24RL) or the regulating valve to make a SLOW reduction (2 pounds at a time to make 6 pounds of initial reduction). With the 24RL, you could go to lap and let the BP leakage make the reduction for you, then put it in maintaining. If that didn't work, we either stopped and found the dynamiter or log-rolled the train- shut off on the throttle and coast. That usually meant 6-mile "braking" distances at 50 mph. That is worst case scenario.

We have 6ET on the steam engine. I have used 6BL, so I should have no trouble adjusting to the 6ET when we get the steam engine going again. We are performing a 20-day boiler wash right now- we have the boiler full, and the water is boiling in the boiler, and after 20 days of chemicals and boiling water, we will drain the boiler and clean out the scale, then retube it. We are also putting roller bearings on our tender right now. Lots of fun!