Push Pull

Discussion relating to the operations of MTA MetroNorth Railroad including west of Hudson operations and discussion of CtDOT sponsored rail operations such as Shore Line East and the Springfield to New Haven Hartford Line

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Tommy Meehan
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Re: Push Pull

Post by Tommy Meehan » Mon Dec 02, 2013 11:53 am

Ridgefielder wrote:I just figured that's simple physics. A Shoreliner weighs something like 100k lbs. A P32AC-DM weighs 274k lbs. Once the train hits the dirt, of course it's going to take the locomotive longer to skid to a stop than the passenger cars-- it weighs 2.75x as much.


This is two different issues. You're talking about braking rates. Other people are speculating the locomotive continued under power pushing the train after the derailment began.

I don't know off hand what the braking distances are for coaches vs. locomotives. I do know that brakes are designed to do things like stop a fully loaded coach (all seats occupied plus standees) in the same distance as an empty coach. They're called self-adjusting brakes. That's so the braking distances remain constant for station stops. So the engineer doesn't have to be looking out the window at stops to see how many people are getting on board in order to know where to begin braking for the next station. Somewhere in here one of the railroaders mentioned that the brakes are set up so the locomotive brakes at pretty much the same rate as the coaches. Brakes are adjustable, they can be set up to give different rates of deceleration.

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Tommy Meehan
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Re: Push Pull

Post by Tommy Meehan » Mon Dec 02, 2013 11:57 am

Ridgefielder wrote:...Once the train hits the dirt, of course it's going to take the locomotive longer to skid to a stop than the passenger cars-- it weighs 2.75x as much.


One other comment. Once the first three cars hit the dirt -- the locomotive didn't hit the dirt -- I would think those coaches are going to slow faster than the rest of the train because of the friction and resistance encountered as they plow across the landscape.

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Re: Push Pull

Post by runningwithscalpels » Mon Dec 02, 2013 12:17 pm

Tommy Meehan wrote: As for the people who died, do we even know that they were in the head car? It sounds like three of them were ejected from the train through broken windows. Did windows only break in the cab car?


In a handful of pictures I saw yesterday (posting from mobile device so can't really access for reference right now) on the river side of the cab car there was a large burgundy splotch on the side. Not saying it's definitive evidence of a fatality, but it certainly looks like someone lost A LOT of blood.
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Re: Push Pull

Post by ACeInTheHole » Mon Dec 02, 2013 12:24 pm

runningwithscalpels wrote:
Tommy Meehan wrote: As for the people who died, do we even know that they were in the head car? It sounds like three of them were ejected from the train through broken windows. Did windows only break in the cab car?


In a handful of pictures I saw yesterday (posting from mobile device so can't really access for reference right now) on the river side of the cab car there was a large burgundy splotch on the side. Not saying it's definitive evidence of a fatality, but it certainly looks like someone lost A LOT of blood.

WHOA. I never noticed that before... holy crap!! That is an unpleasant sight... now having looked at the photo.. There seems to be four guys standing around what looks to be a covered corpse just a bit behind where the blood splotch is. Whoevers blood that was, I do not think they made it.

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Re: Push Pull

Post by Tommy Meehan » Mon Dec 02, 2013 12:33 pm

I saw that too, not sure it's blood. The engineer was in the cab car (of course!) and it sounds like he wasn't too badly hurt.
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s4ny
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Re: Push Pull

Post by s4ny » Mon Dec 02, 2013 12:52 pm

Is it possible that as a result of some system malfunction, when diesel power stopped
the locomotive erroneously took power from the third rail and continued to operate?

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Re: Push Pull

Post by tobeornot2be » Mon Dec 02, 2013 2:25 pm

Ridgefielder wrote:
NH2060 wrote:
Tommy Meehan wrote:
s4ny wrote:...it appears that the locomotive kept on pushing...


What makes you think that?

Probably because the way the middle coach came to a rest zig zagged across the tracks can give the impression that the engine perhaps did keep pushing the consist before it flipped over.

I just figured that's simple physics. A Shoreliner weighs something like 100k lbs. A P32AC-DM weighs 274k lbs. Once the train hits the dirt, of course it's going to take the locomotive longer to skid to a stop than the passenger cars-- it weighs 2.75x as much.


I don't think that's necessarily true. Because the P32 weighs more, its weight could push it into the ground harder and make it slow down faster. The zig-zag could be due to the first coaches hitting the dirt first, slowing down faster than the rest of the consist that remained on the rails a little while longer. Even if the rest of the train was slowing down from the emergency brake application, it's still not going to slow down as fast as a derailed coach, so it's going to be pushing the rest of the train. The zig-zag happens whether the engine is leading or trailing - one example with the engines leading that comes to mind is the 1987 Chase MD collision.

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Re: Push Pull

Post by Ridgefielder » Mon Dec 02, 2013 3:28 pm

tobeornot2be wrote:
Ridgefielder wrote:I just figured that's simple physics. A Shoreliner weighs something like 100k lbs. A P32AC-DM weighs 274k lbs. Once the train hits the dirt, of course it's going to take the locomotive longer to skid to a stop than the passenger cars-- it weighs 2.75x as much.


I don't think that's necessarily true. Because the P32 weighs more, its weight could push it into the ground harder and make it slow down faster. The zig-zag could be due to the first coaches hitting the dirt first, slowing down faster than the rest of the consist that remained on the rails a little while longer. Even if the rest of the train was slowing down from the emergency brake application, it's still not going to slow down as fast as a derailed coach, so it's going to be pushing the rest of the train. The zig-zag happens whether the engine is leading or trailing - one example with the engines leading that comes to mind is the 1987 Chase MD collision.

Good point. I guess what I was trying to refute was the statement that the jacknifed cars were evidence that the locomotive was still under power and shoving the train forward after the lead cars derailed-- and that therefore the jacknifing could be attributed to the train's operating in "push" mode.

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Re: Push Pull

Post by Trainer » Mon Dec 02, 2013 4:49 pm

Ridgefielder wrote:
tobeornot2be wrote:
Ridgefielder wrote:I just figured that's simple physics. A Shoreliner weighs something like 100k lbs. A P32AC-DM weighs 274k lbs. Once the train hits the dirt, of course it's going to take the locomotive longer to skid to a stop than the passenger cars-- it weighs 2.75x as much.


I don't think that's necessarily true. Because the P32 weighs more, its weight could push it into the ground harder and make it slow down faster. The zig-zag could be due to the first coaches hitting the dirt first, slowing down faster than the rest of the consist that remained on the rails a little while longer. Even if the rest of the train was slowing down from the emergency brake application, it's still not going to slow down as fast as a derailed coach, so it's going to be pushing the rest of the train. The zig-zag happens whether the engine is leading or trailing - one example with the engines leading that comes to mind is the 1987 Chase MD collision.

Good point. I guess what I was trying to refute was the statement that the jacknifed cars were evidence that the locomotive was still under power and shoving the train forward after the lead cars derailed-- and that therefore the jacknifing could be attributed to the train's operating in "push" mode.


Even if it is the case that jacknifing was increased under "push" mode, that does not support an argument that "push" mode is always more dangerous. Half the time, the opposite argument could be made as well. Besides, force is force - had "jackknifing" not occured, that force would have pushed the lead cars into the river. Instead, the cars acted like a spring, absorbing the energy from the momentum.

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Re: Push Pull

Post by Patrick Boylan » Mon Dec 02, 2013 5:13 pm

Tommy Meehan wrote:In a derailment don't cars always wind up jack-knifed?

I don't know if they always do, but as others have mentioned, and I've seen photos, lots of jacknifes happen even when the locomotive leads. And beanbag mentioned that maybe the trailing locomotive here breaking the drawbar might have prevented a couple of the rear cars from sliding into cars in front of them.
Tommy Meehan wrote:The irony is, people like that are usually much more reserved in making judgements based on rudimentary evidence.

I'm sure not trying to say I have the answers, I just have the questions.
Tommy Meehan wrote:The railroaders say when a push/pull train goes into emergency a pneumatic valve cuts power to the locomotive. That's an air valve. That's sounds like a pretty basic system. It's the same thing when people were writing that the cab car sends a signal to the locomotive to activate the brakes. That's baloney.

No, it's not baloney. It is a signal in the basic physical science sense. Reducing the brake pipe pressure TELLS or SIGNALS the brakes to apply, because sufficient pressure in the pipe keeps the brakes released. As I said before, it doesn't matter if it's a pneumatic signal, smoke signal, carrier pigeon or dixie cups and strings: doing something at one end causes something to happen at the other end.

Tommy Meehan wrote:The cab car has a brake handle. When the engineer moves the handle it opens a valve that releases air from the brake line. When the engineer opens the brake handle in a cab car you can hear the air escaping. This isn't rocket science.

But if there's some obstruction in the brake line isn't it possible that when the engineer moves the handle that the part of the line after the obstruction might not get its air released, or at least might release more slowly than the part of the line before the obstruction? Isn't it possible that the longer that brake line is, and the more connections it has, the more possibilities there are for the line to have a clog? That's why I'm interested in hearing if the train had any problems stopping at any of the prior stations.
Tommy Meehan wrote:It's also possible if the locomotive was leading the locomotive might've stopped faster than the lead coach and then the cars behind would pile up. People died that way in Chatsworth.

That's one reason why folks like me wonder IF push is worse than pull, not that we KNOW one of them's safer than the other.

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Tommy Meehan
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Re: Push Pull

Post by Tommy Meehan » Mon Dec 02, 2013 6:14 pm

Patrick Boylan wrote:...It is a signal in the basic physical science sense. Reducing the brake pipe pressure TELLS or SIGNALS the brakes to apply...


I don't agree. Reducing the pressure in an air brake line doesn't tell or signal the brake shoes to apply. It causes the brake shoes to apply. When you turn a faucet handle on a sink are you signaling the water to start flowing or are you causing the water to start flowing?

Edited to add - What I was responding to, and without going back and looking for it, was the implication that the brake handle or master controller in the cab car did not directly release air from the brake line. Instead it works by sending a signal through the line that causes the brake stand in the locomotive to activate. That's not the way it works.

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Re: Push Pull

Post by Tommy Meehan » Mon Dec 02, 2013 6:43 pm

8th Notch wrote:Actually you are incorrect on the first half, when you reduce brake pipe pressure you are sending the signal to the control valves on each coach which in turn apply or release the brakes...


How is the "signal" sent? Electrically?

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Re: Push Pull

Post by Patrick Boylan » Mon Dec 02, 2013 7:56 pm

I just want to reiterate, I'm using signal in the physical science sense as we all should have learned and forgotten (cue https://www.edgestudio.com/node/53891?printview=true Guido Sarducci "supply and demand" "como esta usted") from Sir Isaac Asimov, or was it Sir Fig Newton? Every action has an equal and opposite reaction doesn't mean that the brakes are reacting to the pressure reduction anymore than it means they're getting a signal from the pressure reduction. It also doesn't mean they're reacting any less than it means they're getting a signal. Did you know that the brakes are also reacting to the full brake pipe pressure, since it signals them not to apply, which is the same as saying the brakes react to the full brake brake pipe pressure.

Maybe I should have said "tell" instead of "signal".

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Re: Push Pull

Post by StephenB » Mon Dec 02, 2013 8:09 pm

Well, I'm not in the RR business. I'm not even a regular RR commuter. I live in the suburbs of Boston, along the MBTA's Franklin line and what I've noticed is that it seems to me that the ride is a wee *bit* smoother inbound to Boston when we're being pushed (all MBTA/MBCR trains run push inbound, pull outbound) compared to being pulled. It's a slight difference, but I swear I can feel some elasticity in the coupler slack when it's pulled out compared to when it's pushed in. It's not much, but there is a bit front-back bouncing in pull, especially as we accelerate out of Ruggles and down the fastest part of the line. In any case, I can't imagine it making any difference to safety one way or the other.

I understand that freight couplers tend to allow more slack than passenger coach couplers, either now and/or in the past.

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Re: Push Pull

Post by Tommy Meehan » Mon Dec 02, 2013 8:26 pm

8th Notch wrote:Signal as in pneumatic signal, our new equipment here in Boston utilizes fast brake which is elect-pneumatic so sometimes I cross the two.


Was the Metro-North train equipped with that type of brake?

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