Question re: the Physics of a helper engine

[Roger Holmes]

Just gazing out my 7th floor office window at a UP grain train traveling on the Chicago-St. Louis mainline through one of the flattest parts of the world.  There were two engines in the lead, a hundred or so covered hoppers (I didn't start counting in time) and a pusher engine on the rear.  I've been aware of helper/pusher engines for 50+ years but never thought about the following question because I am a student of the Law but have never been a student of the Laws of Physics. 

Query:  I assume (perhaps incorrectly) that engines pulling a train pull the slack out of the couplers as they PULL the load forward hence the cascading clank of the couplers as a stationary train starts to move forward.  Wouldn't the pusher engine push in the slack on the couplers as it PUSHES the load forward?  Is there some point in the train where the load changes from being pulled to being pushed?  I have many things to keep me up at night but don't want to add this to the list.

BTW  I have a Physicist friend who is reading a book about Anti-gravity. He can't put it down..........sorry.

[lock4244]

Better train handing characteristics in being able to start the strain from the head end and the middle. Being able to independently control the remote so that hogger can throttle down the head end and keep the remote under throttle to bunch slack (and vice versa), which is important in flat terrain since there is always ups and downs in the flat lands that can cause slack run in and out leading to a broken knuckle or a pulled drawbar. Ask a CN crew working a 12,000', 15,000 ton general freight about the world of difference it makes. The ability to release the brakes from both ends or from the front and middle yields a faster break application because the loss of pressure originates from two points instead of having to travel the entire brake line. The opposite is true when pumping the air up... pressure comes back faster and releases faster being pressurized from two points.

[rogergperkins]

Roger, I was puzzled by similar situations here in Peoria on Radnor Hill which I can see from my home office.  UP runs coal trains with 2 or more engines at the front and one at the rear. 
I was told that one engineer controls all the engines, like consisting in DCC model rring.
Power at both ends not only controls forward progress, but it also helps against the tendency for the train to speed up going down hill.
  I am a chemist, what do I know about physics?

[Sokramiketes]

It is correct that the slack is pulled out up front and pushed in at the rear. 

One additional advantage to those already mentioned is that rail wear is reduced in curves.

[Hyperion]

Wouldn't the pusher engine push in the slack on the couplers as it PUSHES the load forward?

Yes, if the pusher is really pushing at all, then it pushes in the slack on the couplers.  If it's not pushing in at least some of the slack, then it's, by definition, not really pushing at all because the lead engines are still pulling all the weight. 

Is there some point in the train where the load changes from being pulled to being pushed?

Depends.

On straight, level ground most of the cars should be both pushed and pulled.  No different than if you were, say, tugging on a car to get it out of a ditch and then someone came up behind it and pushed.  You're both doing some work and the car is neither being just pulled nor just pushed, it's both.

As the train goes up-hill it's being both pushed and pulled.  As it crests the hill the cars are being pushed by both gravity (and all the cars coming over the hill behind them) but also being pulled as the lead units attempt to pull the entire train over the crest  Those on the backside of the hill continue to be pulled by the lead units over the hill and pushed from behind.  So both forces continue as the train crests the hill.  Only once half the train is over the hill do things start to change, because then the lead units aren't so much pulling as they are trying to stop the train from rolling down the hill uncontrollably, so it's all pushing force then. 

In theory a DP train is never truly just being pulled with no pushing forces.  In practice it's a little different as the engineer has to constantly modulate throttle settings, so sometimes the train is being more pushed than pulled, or vice versa, and individual cars may be entirely pushed or entirely pulled, depending on the exact forces the locomotives are exerting at that very moment on the train.   

If you can imagine it, it's like moving a slinky along the floor.  You can pull one end and it'll stretch out and eventually drag the rear end if you want -- and you get that if the rear unit isn't pushing enough to keep up.  Or you can push the rear of the slinky, it'll tighten up, and eventually the front will start to slide -- what happens if the rear pushed too hard.  Or you can pull the front, stretch it out, then push from the rear, tighten it up, then pull the front, stretch it out, push form the rear, etc, etc, and that's poor train handling where you keep speeding up and slowing down to keep everything from stretching out or pushing together too much.  Or you can grab both ends of the slinky and move them in unison -- and that's an engineer's goal.  Reality dicates that things aren't quite so perfect, and the slack is always coming in and out to varying degrees, but the goal is to keep enough pushing force to keep the train tight without so much that the rear of the train just pushes the front units along.

So it's kinda like if you were pulling that car and someone else was pushing, the goal is for him to push enough that he's helping you out, but not so hard that he rolls you over with the car.

[rogergperkins]

  The person who answered regarding UP in the Peoria area using helpers told me that "more power is gained" by having 2 engines up front and one in the rear compared to 3 engines up front.

As I said, I am a chemist, not a physicist, and it has been 50+ years since I was studied Sears and Zemanski as an undergrad in physics class.

[Vince Gortner]

Trains Magazine had a nice article explaining all of the pros of distributed power - with good answers on the physics questions! - in a issue sometime last year.   Even on flat land, DPU operations reduces flange/rail wear by doing some pulling from the front and some pushing from the rear, and also add the benefit of pneumatic brake actuation signals coming from both ends of the train instead of just from the front back.   Seeing a lot of UP autorack trains on the Geneva Sub (real, but also Darryl's too maybe?!) in a 1x1 configuration - one lead, one trailing unit.   Not sure I like how it looks but I do see the operational benefits.

[Vince Gortner]

And that was my 25th post!    I have access to the crew lounge now!!!  Woo!

[lock4244]

And that was my 25th post!    I have access to the crew lounge now!!!  Woo!

Heh, welcome to the bacon lounge...

[Bob Bufkin]

He told you so.

[pnolan48]

Do the computer controls of today's engines make this a little easier than in the day of horn signals and just plain engineer expertise?

When I did a pusher up the long grades of my DCC layout, I found that I needed to watch the coupler tension/slack, and adjust the pusher accordingly. It really wasn't that hard. Running the pullers/pushers as a consist wasn't always successful, even with back EMF. The adjustment was usually one click up or down on a 128-step throttle, and wasn't especially time sensitive, as it really took some time for a 60-car train (about the comfortable max) to get stretched out or compacted in.

It sure beat the couplers pulling apart up the ruling grade, leading to a runaway downhill, with a disastrous pancake at the bottom.

[LV LOU]

The person who answer regarding UP in the Peoria area using helpers told me that "more power is gained" by having 2 engines up front and one in the rear compared to 3 engines up front.

As I said, I am a chemist, not a physicist, and it has been 50+ years since I was reading Sears and Zemanski.

Perhaps this is due to the tensile strength of couplers?There's a definite breaking point to a coupler,if the lashup is capable of breaking the coupler of the front car at full throttle,putting one at back will negate this problem,more useable horsepower,same locomotives.Technically,pushers probably can't break a puller while actually pushing,the coupler can only be pulled apart,not pushed to failure..

[GaryHinshaw]

The person who answer regarding UP in the Peoria area using helpers told me that "more power is gained" by having 2 engines up front and one in the rear compared to 3 engines up front.

As I said, I am a chemist, not a physicist, and it has been 50+ years since I was reading Sears and Zemanski.

Strictly speaking, 3 locos with a given throttle setting will deliver the same force to a train regardless of their position within it.  However, on curves the rolling resistance imparted by the rails on the wheel flanges will be very different for the different power distributions, so the net force applied to the train is higher for distributed power (and the rail wear is less). 

The Trains article on DPU's was quite good and worth looking up if you're interested.  I'm pretty sure it was within the past year.

[rogergperkins]

As I said, I am a chemist, not a physicist.  As I also said the person who answered my question, provided that answer.  I did not attempt to validate it.