Train Slack: Where it comes from and why the hogger (engineer) must live with it

You are walking along one of the industrial yard tracks at Marathon. The American Can pulp mill has ten cars of bleached pulp for your train to lift. The snow crystals are squeaking under the soles of your Sorels. You look to the southeast. That feckless sun is cowering close to the horizon this morning because it's so cold. It's not crazy, it's hanging out in Florida where it's warm.

At 0300hr there was banging on your motel room door. You opened it. "Gagnon, Paper Train, 0500hr" "Thanks" If your room had a phone they wouldn't have to drive all the way across Schreiber to tell you when you're ordered. The Paper Train runs east from Schreiber to White River with stops along the way at Terrace Bay and Marathon and maybe others.

So now you've reached Marathon. The tailend crew is doing their thing with the waybills and other paperwork and they'll watch you and the engineman pull by, dragging the ten ancient 40 foot boxcars out of the yard track onto the "main". You've got a Motorola 5 watt portable radio slung over your shoulder. It's a tough battlefield device which is powered by three lantern batteries. Oh, but it's feather light. Well, at least you hardly notice it with all the layers of clothing you have to wear.

You hoist yourself up on the side ladder of the last car. Hold on with the right hand, hold the mike with the left hand.

You press the mike button and say: "OK to go ahead 8616 " The engineer replies: "OK ahead 8616"
(The locomotive leading the other two is numbered 8616 and your train is officially named "Extra 8616 east")

You've got some interesting old diesels today, and the smoke puffs out of them as they start to pull your cut of cars.
All the worn couplers slowly stretch tight and you start moving a couple of seconds after the locomotives start to rev up.
The cold wheels make a ringing sound as they roll along the cold rail.

"Eight cars to the switch 8616" The engineer acknowledges: "Eight to the switch"
(Note to the reader: "Eight cars" means "eight carlengths" - the standard unit of measure for switching.)

"Five cars" "Five"
"Two carsto the switch" "Two"

You drop off the ladder as the cut pulls slowly away from you, the wheels still ringing in the cold.
"OK when you get 'er stopped 8616" "OK to stop "

You swing the derail (DEE rail) onto the rail and lock it. This heavy metal casting locks on the rail so a stray rolling car from the yard would derail upright, rather than running into the path of high speed traffic on the main line.


Marathon Ontario pulp mill with 40 foot boxcars
The pulp mill at Marathon with some old, tired 40 foot boxcars.

railway yard switching at Portage la Prairie Manitoba
On the last car of the cut, you can see a CP Rail trainman enjoying some switching in his T-shirt at Portage la Prairie, Manitoba.


Your instructions have put the locomotives and the ten cars on the main line. Now you can close the track switch to the yard and lock it before you back them up to your train.

You get back on the rearmost car's ladder and key the mike again.
"OK to back up to the train 8616. Fifteen cars to the joint " "Fifteen cars to the joint"
"Ten cars" "Ten cars"
"Five cars to the joint - five" "Five to the joint "

Perhaps leaving it a little late, the engineman makes a quick hard application of the engine brakes to slow down before the coupling.

The locomotives begin to slow from the brake application.
Then ten boxcars (600 tons) want to keep rolling and the boxcar coupled to the locomotive has its coupling go tight after pulling out 3 inches ...
Then nine boxcars (540 tons) want to keep rolling and the second coupling goes tight after pulling out 3 inches ...
Then eight boxcars (480 tons) want to keep rolling and the third coupling goes tight after pulling out 3 inches ...
... et cetera

This was all in slow motion.
It only took a second or so for all the slack to go out of the ten couplings.
The sudden deceleration of the last car (on which you are riding) jerked you hard on the ladder and swung your left shoulder into the car wall as your right hand held on tight.

You are moving at eight miles per hour.
In a few seconds there will be a controlled collision between
"Three cars, 8616" "Three"
"Two cars"  No reply.
"One car" "One"
"Haaalf a car" "Half"
"Okaaay to stop"

From above, the couplers look like this just before they come together at four miles per hour.

diagram, railway couplers: before coupling, knuckles open

The blue-hatched parts are the knuckles in the open position. The "fingers" will strike the opposite flat piece (small arrow). This will cause both knuckles to pivot around a 1.5 inch thick steel "pin" (red "X") until they hopefully lock together.





CRASH!


diagram, railway couplers: the couplers close and push together

The knuckles have closed on each other and look like two hands with fingers curled on each other when viewed from above. The cars are pressed together and the red represents knuckle slack. (You need looseness so a trainman can manually operate the lever to re-open the coupler later when the train is taken apart. Even so, you often have to call "Slack for the pin" or "Slack!" if there is pressure on the coupler when it's time to uncouple.)

"OK ahead 8616" "Ahead 8616"


diagram, railway couplers: testing the coupler by stretching the train

This step is critical. If you don't have the engineer pull forward a few inches to test the joint now, it may fail as you leave Marathon ... you'll be back in the cab ... the tailend crew will be in their nice warm van making coffee ... the engineer will start pulling ... the train will come apart ... the brakes will automatically go on in emergency application ... and you'll take 15-30 minutes to fix the problem and recharge the air brakes in all the cars. Then the Schreiber dispatcher will probably be calling for a chat to see how you're doing as you block his only way of  moving traffic on the Canadian Pacific Railway between eastern and western Canada.

And nobody will be your friend everagain!

In this case, the locking devices (green squares) have held during your test. As the locomotives pulled on your cut of ten cars (against the rest of the train which has its brakes firmly applied) the coupling was stretched. Again, red represents the looseness.

"Good joint 8616" no reply

You couple up the air hoses, carefully and slowly bleed in the compressed air from the locomotives so the engineer can charge and release the train's brakes, and get back into the cab and warm up!


So consider the momentum of all that steel and bleached pulp in terms of Honda Civics:
  • 4000 Honda Civics are sitting, minding their own business, with their parking brakes on (the train)
  • 1050 Honda Civics hit them, travelling at 4 miles per hour (our engines and the 10 cars)
Today, none of our railway equipment will need to have touch-up paint or "permy-sheen" applied.
There was no damage during our 4 mph collision.
Why?

Choose the most applicable answers from this list:
  1. With experiments like this, I must have difficulty getting car insurance .
  2. Railways use very heavy steel to build everything.
  3. Our trainman today was exceptionally skilled.
  4. Special anchoring and padding systems are used within the cars to protect the freight being carried.
  5. The coupling system has built-in protection which is the shipper's best friend ... BUT ... it makes things very interesting for the engineer once the train gets rolling.




Answers 2, 4 and 5 are the most appropriate.


Here is a simplified diagram of the whole coupling assembly:

diagram, railway car draft gear: slack bunched (above), slack stretched (below)
Red arrows and lines show the surfaces receiving strong forces.
When the coupler is at rest, the blue spring expands.

During a hard coupling (top) the coupler is pushed into a spring-loaded pocket (train slack "bunched").
When our locomotives tested the coupling (bottom) the same spring worked in the opposite direction (train slack "stretched").
Very roughly, each car could contribute about 1/2 to 1 foot of slack to a train.

So once the train gets going from Marathon, our engineer really earns his pay because he is dragging a 5050 ton accordion up hill, down hill, and around all the curves. And we have a relatively small train.

If the accordion forces within the train were too great you WOULD need touch-up paint and "permy-sheen" because there could be significant structural damage as big chunks of steel were cracked, bent or broken as the cars' frames were damaged beyond repair and freight (which sometimes does include automobiles) was destroyed. Instead, this happens before major damage occurs:

broken railway coupler knuckle
A broken knuckle.

If 175 tons of force is suddenly applied to a coupler, there is a good chance the knuckle will break, sacrificing itself to save all that bleached pulp or whatever. This one has failed where the pivot pin goes through it.

Left half: If you hold your left hand up to the screen, palm facing you, with your fingers pointing across the screen to the right, your four fingers will be analogous to the chunk of metal on the left. The two mirror-image silver areas represent the break plane of the failed knuckle. The worn silver area  is the "tips of the fingers" part which has interlocked with and pulled against so many other knuckles for so many years before the bad break which ended our knuckle's career.

Right half: On the chunk of metal at the right, the largest silver area is the flat surface which the fingers of opposing knuckles have struck to pivot the knuckle around into the locked position.


When the first half of a train breaks apart (circa 1977), all the headend trainman has to do is:
  • grab his little radio (plus a rubber air hose, air hose wrench, and his battery powered lantern if it's dark)
  • run in knee deep snow on a sloping gravel embankment to find the problem (hose, knuckle, something else?)
  • knuckle? get the engineer to throw one of the 74 to 90 pound spare knuckles off  the locomotive (with a lit emergency flare - called a "fusee" - if you are doing this during a dark Lake Superior night)
  • moving the headend of the train around by radio, get the new knuckle and the break into the same place
  • fiddle around a bit with cotter pins and/or coupler operating levers
  • drop out the remaining broken part from the coupler (BIG thud)
  • watch your toes (sorry, I should have mentioned this step sooner)
  • pop the replacement in and fiddle around a bit more
  • back in the locomotive cab, you don't have to be the engineer's friend for the rest of the trip if you don't want to