I’ve been thinking…
When we took our Amtrak trip, I was kind of feeling for when we were breaking, and when we accelerated.I was also trying to feel the slack action. I really didn’t feel the slack. It just felt like when you come to a stop in a car, although a little less intense.
Any reasons why?
Thanks
Justin
Passenger trains, being relatively lighter and a whole lot shorter, probably have no real reason to be run with built-in slack. Whether you can feel the braking or accelerating probably has to do with an experienced engineer, who’s good at train handling.
Also, don’t passenger cars have some built-in mechanical means between the cars to keep the slack out? Or is that just in Europe and other foreign RRs?
Amtrak trains have tight-lock couplers. They don’t have as much play (and therefore slack) as standard couplers, so trains handle smoother. I think they are standard on most passenger trains today.
When leaving or entering a station with a good engineer at the throttle, you won’t feel much slack.
This observation of mine is true on the MBTA in South Station, I don’t know about other railroads and lines. The MBTA runs push-pull trains into Boston. The loco pushes inbound and pulls outbound. When an outbound train is departing, with the locomotive at the front, you will get some slack action when you start moving. It’s not much, but it is a noticable bump and is loud enough to hear, wheras at an intermediate stop, all you hear is the locomotive throttling up and then you start to move.
I theorize that South Station is built on an incredibly minor grade, and when the handbrake on the end car (the cab control car on the inbound run previous) is released, the train drifts backwards as much as the little slack in the couplers allow it. That, combined with the locomotive starting to pull forward, creates the slight slack action I’m feeling.
Also, the power-to-weight ratio of the train is way less than for an automobile. The car is maybe in a range of 60 to 200 HP per ton, whereas the train might be from 4 to 10 HP per ton, depending on the number of engines that are ‘on-line’ and the number of cars and their weight, etc. So the train accelerates far slower - and that far lower acceleration rate means that the ‘jerk’ that accompanies the start and run-out of the slack is way less in magnitude. When braking, another reason is that the Amtrak cars have a very sophisticated braking system. I’ll leave it to antoher expert to explain in detail, but I believe they are a disc-brake deisgn that adjusts for the car weight and modulates the rate of application. Also, being a shorter train - and with a higher train-line air pressure, 110 PSI vs. 90 PSI on freights ? - the air brake response is a lot snappier. Finally, to give the most credit where it is due - the train-handling experience and skill of the engineer is paramount. ‘A jerk can cause a jerk’, but it takes a pro who takes pride in his/ her craft to do it smoothly, each time, every time - but you already knew that, anyway, right ? - Paul North.
Say what…You are going to haft to exsplain this to me. and i will make it simple forget service make it all frieght or all passenger service then tell me how 110 vs 90 psi makes brakes snappier???
All the technical and physical reasons above are 100% correct. But let us not overlook the skill of the engineer. In the old days, of course, making the passenger board was a badge of honor acknowledging one’s skill and longevety. Even today where you don’t have decades as a fireman and more as a freight engineer before entering passenger service, skill in knowing the engine, the train and the railroad goes a long, long way in making the ride as comfortable as possible for the passengers. One key in starting a train that I know is, as engineer, stare straight down at the ground as you slowly release the brakes and open the throttle to tell when you are moving. I am sure there are some real engineers and runners here who can maybe, and hopefully will, tell you more.
Arrgghhh [oX)] - you got me there, matey ! wabash1. [D)]
I was thinking of the shorter train length when I wrote the ‘snappier’ part, and then went back and dropped in the PSI part without editing it. Then I got to thinking about it a little while ago while wandering down the hallway and I too thought - Nah, the higher pressure doesn’t make the brakes work any faster.
Maybe what the higher pressure does give you is more of a possible reduction before the cylinder pressure equalizes and hence more braking force and a faster stop - but that would only be in a Quick Service or Emergency applications, which are not involved here since we’re discussing smooth and slack-free braking.
Sheesh - don’t I get at least partial credit for knowing about that pressure difference ? [;)]
Paul,
I will give you half a mulligan. You are correct about higher brake pipe pressure being carried on passenger trains than on freight. Snapier response is due certainly to much shorter trains, and perhaps to more responsive control valves. They latter is speculation as I am not a passenger guy.
Mac
Paul
I am not busting your chops I just thought they did something new and not told me about it… The differance in air pressures ( and why cut frieght back is stupid) due to the way you apply and release, its how much you have to work with. on passenger trains you get 15psi to slow then release 6 psi then reapply 10psi and stop ( just as a example) on frieght there is no slight release when i get 10 psi and slow to much i haft to release it all, then re-apply and i haft to get a let more than 10 to apply because i was not charged up totally. when re-applying you draw down from the main reserve and then release it takes air to re-charge the system and it takes time this is why frieght engineers in mountain areas are some of the best engineers you will ever see.
As far as credit goes thats what banks and stores give you around here we all gain wisdom, and hopefuly get smarter learning something. Even I have learned things on here thats what its about.
Speaking of starts so smooth that you have to be looking out the window to know you are moving, how many of us remember starting with half a dozen or so head end cars–Railway Express and storage mail? If the engineer were not extremely careful, you would be aware of the start, even with your eyes closed. And, there could be a definite jolt when the train stopped, as well. There were no tight-lock couplers on those cars.
Johnny
Our south departure is on a slight downgrade. Engineers get us rolling by simply releasing the breaks and the beginning of the motion is almost imperceptible.
Starts elsewhere are frequently a matter of how the engineer stopped the train - slacked or bunched. With a variety of engineers (mostly volunteers), you rarely get the same ride two days in a row.
Yes, as a volunteer, let me tell you that each different car in an “historic” consist brakes differently. Usually an 7# reduction on an up-hill stop will keep the train stretched until a stop. Just at the time of the stop, set the throttle to idle, set the independent, and release the automatic. In the ideal this will leave the train stretched (the cars’ brakes stopped the train and the locomotive pull kept it stretched). Because our cars range in age from 90 to 50 years old, there is a wide range of braking force and weight of the cars. Slack occurs when some cars do not apply drag and catch up with the stopping train. As soon as the automatic is released, the bunched up cars move back and give a little tug. If stopping on the level that slack won’t run out until we start up again. In theory, there should be no slack if all cars stop with approximately the same braking force. If the force is unbalanced some cars will bunch up and then “unbunch” when we start back up again. I am sure that MBTA and Amtrak cars brake more uniformly so the slack doesn’t run in. And, the tightlock couplers (not the brakeman’s friend – they just don’t couple nicely to other couplers) have less slack than the standard couplers – so that the slack run in and out is far less on passenger cars than freight cars.
Back in the day of Railroad operated passenger service, I know some carriers had their passenger service equipped with electro-pneumatic braking systems. Similar in concept to ECP braking that is attempting to gain a foothold in freight train braking. The applications and releases were transmitted by electric signals, thus the brakes were commanded to operate at the speed of light (electricity) vs. the speed of sound (pneumatic). In the event of the train not being fully equipped for electro-pneumatic operation, the ‘natural’ pneumatic operation of the brake valves occurred. Does Amtrak utilize electro-pneumatic braking systems?
Tight lock couplers really don’t have anything to do with slack. The reason you don’t have much slack action in a passenger train is because of the spring actuated buffers on each car (the plate at the bottom of the diaphrams) which keep the couplers in tension.
When I rode the Rocky Mountaineer from Calgary to Vancouver I noticed times when you would feel slack when starting and stopping and sometimes when you wouldn’t. I think both Tyler and Larry hit the right spots in their posts about whether there were grades at the stopped points and the skills of the engineers involved.
However, having ridden trains from the age of 3 days, I was attuned to the noises a train makes before you would hear or feel any slack action. Thus I would be waiting for the sensation before it happened, and on that trip there were no significant bumps. I’ve been thinking since yesterday how to describe it, but all I can say is when it happens, you know it couldn’t be anything else. It is a very slight change in sensation from moving to not moving or vice versa.
It is not the same sensation as you would feel in a car. When I did feel slack action you knew you were in a 70 or 80 ton railcar and not a two ton automobile. You just feel like you are part of something more substantial.
Justin, I know this post raises more questions than it answers, but as you ride more trains one day you will pick up on it, and from that point forward it will be obvious.
AgentKid
In fact, the Amfleet cars were delivered with EP braking, but it was removed because the benefit in stopping distance was so slight. I think I read somewhere that the AutoTrain has ECP braking, though. The “snappier” application with 110 psi vs 90 - all other things being equal, 110 would get the brakes on a hair quicker than 90 because the greater pressure differential between the reservoir and the brake cylinders means a higher flow rate and faster pressure build up in the cylinders.
Back in the old days, passenger trains almost always stopped stretched, letting the cars drag the still motoring locomotive to a stop. I can remember absolutely imperceptible starting of GG1 hauled trains. However, now I believe Amtrak trains (NEC, at least) use blended braking where the loco DB is used at higher speed and the braking system transitions to air as speed slows. What I don’t know is how you avoid the “bump” bunching them up and then later stretching them out. Anybody?
Hey, wabash1 - I knew you weren’t busting on me. We’re cool. [8D]
Now I’m glad I mentioned it, because you give 2 excellent reasons/ examples of how and why the passenger brakes are different:
Speaking from the consist, not the seat box…
The relative smoothness inherent to the inertia involved has a lot to do with it. During one discussion with an engineer, he mentioned how, when starting from our station he can feel the cars, which are usually slack when we start rolling, “come in” on the engine - ie, catch up and bunch up. Bump-bump-bump-bump. We don’t feel that in the cars, though I suppose we might if we were really paying attention to it.
One train-handling headache we have on every single run is the spot we call the “Camel’s Hump” - depicted below. The grade on both sides is about 1%, so we’re pulling hard on the way up, and setting brakes as we crest the hill. Unless we’ve got a newby at the throttle, I never notice the transition on the consist, even though sometimes the engineer is making an application (and bailing off the independent) and sometimes he’s setting up the dynamics.
Of course, there was the time I was nearly thrown off my feet by a green student engineer who took just a little too much on our RS3’s 6 brake as we started down the hill…
