Why cant trains stop immedietly?

Question

Why cant trains stop immedietly?

Answer 1

too much weight behind the force of movement

Answer 2

Trains cannot stop quickly because they have little traction in relation to their mass.

Answer 3

Inertia.

Answer 4

Nothing can stop 'immediately'. Just think cars. thinking time, braking time. The forces involved in stopping anything 'immediately' would probably kill everyone inside.

Answer 5

Momentum..

Answer 6

The same reason NOTHING that is moving can stop immediately - heck even a bicycle can't stop "immediately", and in a sudden stop what happens? You fly over the handle bars because you are still moving and there is nothing to r****d your movement. As another poster suggested, look into Newton's 1st and 2nd laws of motion. Mass, weight, speed all play into stopping distance.

Answer 7

Some good answers here, but.....

The issue is more complicated than the weight of the train.  It also has to do with the force generated by the brakes.

For example, take a simple automobile.  Let's say the coefficient of friction of the rubber tire on the asphalt is .5.  That means if 1000 pounds is on the tires, they will not slide until a force or 500 pounds is retarding the motion of the car.  Once that's exceeded, the tires slide.  If the car is loaded to 3000 pounds, the slide would start at 1500 pounds.  But, in both situations, the stopping distance would be the same.  More weight, more braking. but the proportion is the same.

If steel wheels slide, the flat spots create considerable damage to the bearings, and the rails.  They must be protected.  Therefore, the braking ratios are held low to prevent skids.

But, in addition, the problems of slack and brake propagation prevent higher ratios.  The brakes are applied when the engineer reduces pressure in the air brake supply.  It takes time fo this reduced pressure to propagate to the rear of the train.  If the brake ratios were higher, the front of the train would be stopping much faster before the rear even starts to break and the rear would derail pushing against the front.  

Even with current conservative ratios, emergency applications from the front have caused secondary derailments in the rear.

I believe currently, the limt on a freight car is .13 of gross weight, and with the shoe coefficient of about .30. For a 100,000 lb car that would be 100,000 x .13 x .30 = 3900 pounds.  

If the coefficient of friction of the steel wheel on steel rail is about .25, theoretically 25,000 would be the threshhold of sliding.

But current rule limit empty ratios to .30.  (It was .35 fo years, but they were getting too many slides. )   This means the practical limit is 100,000 x .30 x .30 = 9000 which means coefficient of friction as low as .09 is the worst frequently encountered in service.

This is the full service braking ratio.  The emergency is about 120 % of that.

So, there are two problems, the slide problem, and the propagation problem.  The latter is what keeps the braking ratios low for safe operation.

Passenger equipment, however, has anit-lock brakes, and much higher ratios can be tolerated.  

But as I'm sure Hoghead will attest, locomotives have had "anti-lock" brakes for a long time.  The computer is the engineer's brain.  If he is operating light locomotives and the independant brakes are applied and wheels skid, the engineer will get an alarm, or feel the slide, and reduce the braking with his hand on the brake lever.

Answer 8

I'm glad "flyboss" decided not to go mean.

Newton aside, think of it this way.  Your average freight train is equal to three or four WWII era US Naval destroyers on roller bearings.  It takes a long time for mass such as this to be brought to a stop, especially if it is moving at any appreciable speed.

If you would like more in depth information, I would invite anyone to visit my 360 and check out the Hoghead's Highball blog, in particular the series titled "The Air", which has some useful information for the curious or the professional.

Answer 9

A typical train consisting of 2 engines, and 100 cars at 137 tons a piece is roughly......28 million pounds.  That's alot of weight.  All of the answers above have their points and are correct.  Running and stopping a train takes time.  Even if you were to enable an emergency brake application....where you put full brakes on right away on every car,.........you're going to keep on sliding even though the wheels may not be turning.  Same as sliding in a car....except we do it on steel.  It's just to much momentum, weight, inertia.....all of that good stuff to stop it in it's tracks.  There's only one thing I have seen that will stop a train immediately..........and that's a head on collision with another train of equal size............and it's not pretty.

Answer 10

Weight is the main reason it takes trains so long to stop.  Locomotives weigh anywhere from 120-200 tons, and rail cars can weigh anywhere from 30 to 150 tons.  Count how many locomotives and cars are on a train the next time you're stopped at a crossing, and you'll get an appreciation of the size.

On top of this, air brakes on a locomotive and rail cars don't apply immediately, or at least not as quickly as an automobile.  From the time the engineer sets up the brakes, it can sometimes be several seconds before the last car on the train has the brakes applied with any significant force.

Answer 11

The answer to your question lies in Newton's First and Second Laws of Motion, but to put it in the simplest terms, because they weigh a lot

Answer 12

Yea I know    My grandson had his stop on a dime when his sister pulled the plug from the receptacle to play with her lite brite.

  HERE'S YOUR SIGN.  :)

Answer 13

The BNSF Engineer has the most correct answer so far.  I am also a railroad Engineer.  Even when you put a 55 mph train into emergency it will still take over a mile to stop.  55 mph passenger train will take about one mile to stop, and a freight train will take a lot longer distance to stop.  He was also correct at saying the only way to stop a train almost immedietly is by a head-on collision.  They only thing bad is not only will you more than likely not live through it but if you should live through the impact then you still have all of the weight behind you that is still moving how ever fast you were moving to begin with.

Answer 14

becuase of inertia

intertia is the tedency of an object to keep its state of velocity

since the train is very heavy, there is also a lot of intertia . all that weight wants to move forward and you have to stop it . that would take a while

Answer 15

I could go mean...but I won't....

The answer is the weight of the train combined with the steel-on-steel of the wheels and tracks....

it's just impossible to stop that much weight 'immediately'

think of this:  when you are in a boat, can you stop immediately?...no...the reason for THAT is that there is nothing to stop the weight, because of the lack of friction available to stop you..

a CAR cannot stop immediately, either...it just takes less than a train, because it is made differently to handle less WEIGHT...

hope this is clear for you...