
How Do You Get a Train Moving? - zw123456
http://www.wired.com/2014/06/how-do-you-get-a-train-moving/
======
femto
It seems like cheating that the answer to "How Do _YOU_ Get a Train Moving?"
is "With a locomotive."

I'd answer "with a crowbar". I used to work in a railway museum and when we
didn't have an engine in steam we'd do our shunting with crowbars. The
technique is to get a 6 foot long bar with a bent end. Wedge the point of the
bent end between the tread and rail, so the "elbow" is resting an inch or two
behind the wheel and the bar is up at a 45 degree angle, so forming a lever
with a lot of mechanical advantage. Put all your weight on the bar. Eventually
the car will start moving. As the car moves, release the bar, slide it forward
and repeat. Eventually the car will be moving fast enough that you just run
along beside it, sliding the bar forward and jacking up and down with your
hands. One person can move a typical car on the flat, but if that's not
enough, a person with bar on each wheel will get most things moving.

Always have someone on the handbrake, as you will kill yourself if you try to
stop the car with the bar!

~~~
ohwp
Other answer: with a lot of model trains: [https://www.youtube.com/watch?v=vC-
dmKHiCnY#t=445](https://www.youtube.com/watch?v=vC-dmKHiCnY#t=445)

~~~
Luc
According to the video about 100 kg tractive force is enough to get the
locomotive moving. That's a lot less than I expected (though I've seen a video
of a strongman called John Massis pulling a loc by his teeth).

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DINKDINK
This is a very poorly written article (I'm amazed this is written by a physics
professor). Static and kinetic friction is only a resistive force to a train
if there is slippage between the wheels and the track.

-Assuming a no slip condition at the wheels, the friction will rotate the wheels not drag on the train/car.

-The reason why there is a gap/space in the train couplings is a tolerance/GD&T issue.

-Engine torque is almost always higher at lower speeds [http://image.circletrack.com/f/enginetech/ctrp_1003_nascar_s...](http://image.circletrack.com/f/enginetech/ctrp_1003_nascar_sprint_cup_engine/27509422/ctrp_1003_21_z%2Bnascar_sprint_cup_engine%2Bdyno_sheet.jpg) (this is why you have to down-shift when climbing a hill) and is what gets a vehicle moving.

Top commenter got it right:"The locomotive is accelerating itself and the
cars, not overcoming the friction in the bearings. Axle bearings have pretty
low friction (at least negligible compared to the inertia of a loaded car); if
they didn't, they would get hot, the grease would melt"

~~~
msisk6
Slack is something railroads would love to get rid of.

If the slack isn't managed properly when starting a train the jerk can rip the
coupler knuckle right off and split the train (which brings everything to a
stop when the brakes automatically apply when the air supply hose is
separated).

I've seen many a train startup on a grade with all the slack stretched out and
wrapped around many degrees of curvature: modern diesel-electric locomotives
have tremendous tractive effort right from a standstill.

These modern locomotives have computer-controlled wheel slip systems, sand
dispensers (that inject sand between the wheel and the rail to increase
friction), and active steering trucks. And for the past decade or so
locomotives have switched from DC traction motors to phased AC (which don't
have high-current brushes that can melt at high-load, low-speed).

All US railroads have mandated low-friction roller bearings in everything
since the 60s or 70s, too.

I'd imagine the railway engineering field has all the physics of this stuff
well-documented since the basics of railroad operation hasn't really changed
in 100+ years.

~~~
jessriedel
I can't understand what your reply has to do with DINKDINK's comment.

~~~
VLM
I would have been a 4th generation RR employee, if the RR hadn't gone
bankrupt. Got to spend all kinds of time as a kid in rail yards going stuff
that would probably be illegal for everyone involved now a days. This is
before CCTV everywhere and extensive tracking. Anyway I'll try to match up
first post comment/question with second post correct answer, if this reformat
helps:

Torque is higher at low speeds - No, with "modern" say post 1960 diesel-
electric locos its constant. Or more accurately current limited to a max which
is achievable at almost any reasonable low speed condition. Maybe a car
analogy is all locos can "spin their tires" under any condition up to 45 MPH
or so. There are no "econobox locomotives" or the equivalent.

Peculiar spherical cow assumptions about no-slip conditions - No, its very
complicated and as you'd suspect lots of anti-lock anti-skid technology
actually started on trains before it went to cars, and trains are always
slipping a little (not very much, and the optimum is not exactly zero just
close to it)

Slack is an intentional mfgr tolerance thing - No its something that could
(almost) be eliminated and is a huge PITA under certain operating conditions.
Its goal zero not goal 0.1% by design or whatever.

Theoretically the bearings must be pretty good - yes indeed they are, awesome
roller bearings required by regulation since the 60s

If you provide specific examples of what doesn't make sense it would be
possible to expand upon that specific topic.

~~~
sounds
To start a train, is it the coupler springs not slack which allows each car to
start rolling? (fighting momentum, not so much static friction)

~~~
VLM
OK... first of all there are thousands (no joke) of coupler designs. If you've
seen "Thomas the Tank Engine" you're familiar with 1830s era chain/buffers but
those are obsolete. I can only talk about the modern USA Janney coupler. My
comments might be totally wrong in Norway or Russia or pretty much anywhere
else. Saudi Arabia of all places uses Janney couplers. (edit to add my point
is there may exist a coupler design in Germany or England or something that
inherently uses springs. Just not in the USA, not for over a century, ours are
solid bars of interlocking hands made of steel with a guard that prevents them
from unlocking and a pin/latch arrangement that prevents them from swinging
open...)

Anyway if its under tension it can't be unlocked. When you release tension you
can shove a pin out of the way (if its not rusty) and it then can be unlocked.
So that in itself is an interesting comment. If the train isn't under tension
it literally can't fall apart short of metallurgical failure (like a crack)
and train crews don't like it when their train falls apart, so the don't like
their train not being under tension. All freight couplers are the same but the
hazmat and passenger car couplers are a little weirder and slightly harder to
uncouple and have a lot less slack by design (I've been told its mostly closer
mfgr tolerances). None the less couplers do rust and removing tension means a
rusty coupler could unlock and train crews hate that. So yeah, crews don't
like a train that gets compressed. You have to back up to switch but you don't
do it for fun or whatever. Aside from the whole visibility thing which means
you could kill some poor dude a half mile away backing over him and never know
it.

The thing that connects the coupler to the center of the car is called the
draft gear. To say it has a high spring coefficient would be an
understatement. Some cars/engines don't have any draft gear at all and the
coupler is welded right to the center / spine of the car. When a train
couples, the spring flexes so things aren't permanently bent. Its not like a
shock absorber in a car which flexes all the time while rolling along, its
more for damage reduction (edited to add, and the coupling process itself)

The slack comes from the 1/2 inch or so space in the coupler, times a large
number of cars. A hundred coal cars times 1/2 inch is like 5 feet of slack ...
So no need for springs you've got all the slack you can handle in the
couplers. You could spec a coupler with 1/16th inch precision but then every
time you switch a freight car you'd need a guy with a sledgehammer to screw
around for 10 minutes so thats hazmat and passenger only which I have no
experience with.

Since the (19)60s all cars have roller bearings and the static friction is so
low that unless you applied the brakes in some weird manner the cars will
kinda flop around on their own. Pushing a train is an interesting experience
that should be a part of all physics experimental curricula. You really can
push a train by hand. It won't be easy and it won't go fast and it definitely
won't accelerate very fast, but its not hard. So you could only guarantee
compression if you're actively moving / just moved or you're headed downhill
slightly. Wind can actually move cars around. This is why all cars need a
parking brake (that horizontal wheel) in addition to the air brakes. It would
be mechanically complicated to put a train under compression but it could be
done by locking the brakes or something.

The static friction of a car is like 100 pounds if the bearings are in good
condition and the parking brake is completely removed etc etc. The slightest
incline, or maybe even wind load, exceeds the force of static friction. If you
have a hundred tons of coal in a car, the static friction is a mere rounding
error. It might be calculable, or even measurable, but it won't be noticable
when driving.

(edited to add the best car analogy I can give is its like a skateboarding kid
hanging onto the back of your car, you're just not going to notice the extra
friction compared to the mass of the car)

~~~
sounds
Realizing that you don't have as much experience with passenger trains, but
just out of curiosity about the physics --

On a theoretical passenger train where there is exactly zero coupler slack...
say this train travels on a perfectly straight and level track so there's no
need for coupler slack at all, maybe the cars are actually welded together for
this experiment --

Would it be particularly difficult for a single engine to start the train?

(Assume the train isn't ridiculously heavy or anything. This is really just
curiosity about whether slack and/or draft gear make a difference when
starting from a standstill.)

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js2
Huh.

[http://en.wikipedia.org/wiki/Slack_action](http://en.wikipedia.org/wiki/Slack_action)

"Loose coupling is necessary to enable the train to bend around curves and is
an aid in starting heavy trains, since the application of the locomotive power
to the train operates on each car in the train successively, and the power is
thus utilized to start only one car at a time."

An example at 2:20 in this video:

[http://www.youtube.com/watch?v=9D9zE6mju48&t=2m20s](http://www.youtube.com/watch?v=9D9zE6mju48&t=2m20s)

~~~
refurb
So how do you get a locomotive started when the engine and cars are not on
flat track? If there is any kind of incline, you'll have no slack in the
couplings.

Can you use the brakes somehow?

~~~
tempestn
I'm guessing you answered your own question. If they put the break on the last
car, they could let the train compress backwards before starting. Naturally it
would still be more work than starting from flat though, so obviously there
would be an incline limit beyond which a given train couldn't start
regardless.

~~~
refurb
Can the last car's brakes hold the weight of all the cars in front of it? Can
the conductor apply each car's brakes independently?

Interesting stuff!

~~~
tempestn
The original article stated that a train wasn't able to start because the
caboose's brake was on, so presumably the answer to the second question is
yes, at least for that car. Presumably the first question would depend on the
slope of the incline (although if necessary other brakes could be engaged
after their cars' couplings were compressed). Anyway, all just conjecture on
my part!

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rb2e
From my father, who used to drive steam and diesel locomotives in the UK on a
Heritage railway (2ft Gauge).

He says strictly speaking, legally you're not allowed to reverse a train once
passengers are inside without permission, but you can "rock" the locomotive
forwards & backwards if you're stuck.

Procedures may differ in the USA.

~~~
VLM
Also the reasoning is you can derail a collection of (relatively) lightweight
cars by pushing behind them with a very heavy engine, but its much harder to
derail by pulling.

Think of trying to keep a rope straight by pulling on it, vs trying to keep a
rope straight by pushing on it.

The passenger train rule is because its hard to kill people by tipping over
coal cars especially when its a rare failure mode anyway. But you wouldn't
risk it with cars full of passengers.

(Oh and edited to add WRT switches, its subjectively 10 to 100 times harder to
derail on a switch going one direction than the other, so again you'd risk
backing a bunch of coal cars thru a switch but it would be pretty dumb to do
it with cars full of passengers)

------
symmetricsaurus
I am fairly certain that trains do not need the slack between cars to get
moving. Like others have pointed out, trains seem to be perfectly capable of
starting on inclines where there would be no slack.

There is one assumption in the article that is certainly wrong: "...but it
seems crazy to think that the train’s friction coefficient is 10 times more
than the cars"

This does not make any sense. The forces we are comparing is the friction
between the rail and the wheels in the case of the engine and the friction in
the bearings of the axles in the case of the cars. There is of course some
frictional force from the deformation of the track and the wheel as well but
as they are both quite rigid (being made of steel) it should not be very
significant.

~~~
hvm
The article has the physics completely wrong.

Static friction is what opposes movement between two surfaces that are static
next to each other and kinetic friction is what slows down movement between
two surfaces. Static friction is always >= kinetic friction.

This is explained very well in here (the top picture says it all):
[http://hyperphysics.phy-
astr.gsu.edu/hbase/frict2.html](http://hyperphysics.phy-
astr.gsu.edu/hbase/frict2.html)

Think about how when you start your car from a standstill you need to be
careful about your clutch release but when the car moves even the tiniest bit
you can even start in the second gear if you want.

So, the engine car's wheels get kinetic friction (because they're rolling) and
the other cars' wheels (which are stopped) are blocked by static friction.
When you have slack and you move one car at a time you only need to beat the
static friction of one car at a time. Even if the difference is 10%, if you
have 10 cars and it starts to add up. I'm not saying slack is required for
starting the train, just that there is an increased difficulty in doing so
without slack.

~~~
hvm
Also, this is why doing a wheelspin is never a good idea when you have trouble
starting your car/train. The traction when the wheels are blocked but the
engine tries to move them is always a lot better than when they're spinning in
place.

~~~
Gracana
To be super-pedantic (because the topic is interesting), a little spin is good
for an automobile. Rubber friction is a combination of adhesion, mechanical
deformation, and wear.[1] In order to maximize traction with a tire you will
have some optimal slip, increasing the "wear" component beyond what is seen in
"normal" driving conditions. Of course, too much sliding/wheelspin and you
reduce the other components. It's a balance that depends on rubber compounds,
tire construction, and road conditions.

[1]
[http://insideracingtechnology.com/tirebkexerpt1.htm](http://insideracingtechnology.com/tirebkexerpt1.htm)

------
foobarian
Of course modern trains have electric motors on each car, and engines are just
giant diesel generators making the electricity...

~~~
comrade1
The trains in the u.s. are still pretty primitive. Do a google image search on
amtrak for some cringeworthy pics.

Where I live in Europe the only trains where the engine pulls are freight
trains. All passenger trains except some cog-wheel trains in the mountains
have diesel electric generators and individual motors on each wheel.

~~~
tanzam75
While Switzerland has opted to go all-EMU for its passenger trains, that's not
true of every country.

For example, the only powered axles on the French TGV are at the front and
rear of the train. All the axles in-between are unpowered. The TGV isn't
exactly a primitive train.

The TGV was the basis of the Spanish AVE and the Korean KTX. The KTX also uses
additional powered axles on the first and last cars, but it still has a lot
more unpowered axles than powered ones.

As for Amtrak, the Acela isn't a primitive train. It's a mashup of the French
TGV and the Canadian LRC. And yes, it has unpowered axles in the middle. Its
main problems are its excessive weight (forced by FRA regulations), and the
totally inadequate investment in the track over the last 80 years. The basic
design isn't that bad.

~~~
iSnow
>The TGV was the basis of the Spanish AVE

Well, the AVE 103 is based on the Velaro platform which most certainly has
powered axles on every car, as you would expect from a German train. The same
goes for the Russian Sapsan, the Chinese CRH3 and the German ICE3

~~~
tanzam75
The AVE 103 may be based on the Velaro, but the AVE 100, 102, and 112 are
based on the TGV, which has non-powered passenger cars.

The point is that non-powered passenger cars do not automatically indicate a
"primitive" rail system, as the previous commenter seemed to think.

Some countries have made one choice, other countries have made the other
choice, and some countries even use both technologies simultaneously.

And that's just high-speed trainsets. It's quite common to find non-powered
passenger cars on conventional trains as well.

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epx
I think that having to appeal to couplings' slack to be able to start a train
was used in the steam era, because steam engines are constant-force machines
(limited by steam pressure and cylinder area). Diesel-electric or electric
machines can start stretched trains, even manuals of 1950 engines advise to
start stretched. Starting slacked is dangerous because of the impacts on
couples (current trains are much, much longer than steam-era trains).

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lutorm
The article totally misses the fact that the static friction to overcome on
the cars are a static _torque_ not a force. The forces on the cars are acting
on the coupling and at the wheel contact point, but the static friction to
overcome is at the axle diameter. The torque arm is such that the force there
is larger by the ratio of the wheel and axle radii. The larger the train
wheels, the less effect the static friction will have.

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NN88
ALWAYS wondered this...

