Why do the fast trains take a mile and a quarter to stop in a emergency stop?

Question

The fast trains with a funny front take one mile and a quarter to stop when the emergency brake is on

Answer 1

You try stopping something the weight of a train using disc brakes whilst the wheels are smooth steel running in equally smooth steel rails - and that's assuming the rails are dry and not covered with squashed leaves ! It takes rather a long distance....

Stephen A please note that on this type of stock each brake cylinder has its own feed of air, electrically controlled. They're not old fashioned Westinghouse brakes anymore!!!

Answer 2

It has a lot to do with the weight of the thing! Take a plane coming in at about 250 miles an hour, even with engines in reverse thrust the spoilers up and the brakes on, it can take them a mile and a half to stop! That is rubber on concrete! Trains use metal tracks to guide a train with metal wheels at high speeds. In England the fastest trains are the East coast mainline 225, they travel at 140 miles per hour but they can stop in 1 and 1/4 miles under an emergency. It is like a car doing 70 down the motorway, you can't stop that in an emergency in the same distance as one doing 30!. One word for all of this is inertia!

Answer 3

Again, the answers above are "partially correct", so here are a couple of corrections.

The air brakes do not operate by putting air into the system. The brake system starts as a "charged system" and brake applications are made by letting air out of the system, with freight trains carrying 90 psi in the brake pipe and air reservoirs and passenger trains carrying 110 psi. When the locomotives are placed on a train they charge the air system from the main reservoir, which carries 140 psi, nominally. Sometimes the train is pre-charged utilizing "yard air".

As the air is exausted from the brake pipe, the control valve on each car puts air into the brake cylinder from the auxilliary reservoir, for service applications, and adding the air from the emergency reservoir as well, in emergency applications. These reservoirs are on each car.

One of the things that makes a major contribution to stopping distance is the propagation time of the brake application, which is (rounded off) 500 feet per second at a service rate, and 1000 feet per second at emergency rate. Lets assume a train length of 7500 feet. At a service rate, it would take a full 15 seconds before the brakes even begin to apply on the last car. Assume a speed of 60 MPH, then the train has covered a quarter mile before beginning to slow down. Not much better in emergency as a great distance will still be traveled. These are not slow acting brakes. This is simply the way they operate, as envisioned and designed by George Westinghouse. Though the materials and operation have been improved, the physics are still the same. The brake equipment of today is state of the art, in both passenger and freight operations.

Now factor in TPOB (Tons Per Operative Brake). This a simple average of the total trailing tonnage divided by the number of brakes (cars) in the train. The higher the TPOB, the greater the stopping distance.

A typical train would probably in the neighborhood of 8,000 tons, with perhaps 110 TPOB or slightly more. Of course unit trains are usually much heavier, but your everyday average is represented here.

The trains you describe with the "funny front" are high speed passenger rail, and their stopping distance is longer due to higher speed, as their tonnage is much less. In this instance, the law of intertia is a major player, even with trains with disc brakes and the "blended brake" of passenger diesel electric locomotives.

The bottom line is, if I can see you, it is already too late to stop for you. So, make everyones day and stop, look and listen before crossing any tracks, then HEED the warnings.

Answer 4

Because actually, when you pull the emergency brake, it is the driver who brakes for you. So even if you pull inside a tunnel, the driver will simply wait until he exits and then stop. A train travelling at 140 km/h (90 miles/h) will take 1 kilometre (0.62 miles) to come to a complete stop, because the train needs to deaccelerate and if it were to stop in a less large distance the train would have to brake much harder and that would be dangerous for passengers. A train (a TGV for example) travelling at 300 km/h (186 miles/h) can stop in less than 2 kilometres (1.2 miles) but braking any faster is too dangerous.

Answer 5

Actually the fast trains also have electronic controlled brakes, that react a lot faster. Some high speed trains have as many as four disk brake rotors per axle, and some have eddy-current or magnetic brakes, powerful electromagnets against the rail which develop retarding force, as well as create magnetic force which adds to gravity to create a greater normal force and allow more braking without skidding wheels.

But most of all, Energy is Mass times Velocity Squared. The higher the speed, exponentially more effort is required to retard it.

And, no one asks why it takes them so long to get at that speed in the first place. That's why they limit the number of stops high speed trains stop at. It takes a while to slow down and speed back up.

Answer 6

Ok I drive a big truck for a living... to put things in perspective the average car on the road is about 3000lbs, the truck I drive is 144,000lbs. For me to stop my truck from 60mph in an emergency will take about 1/4 mi ( this is why you don't cut off semis by the way... crunch crunch crunch) Now in an ideal world I would be able to stop much sooner but lets look at the math for a sec... 3000lb car four brakes = 750lbs a brake to stop a car and most cars have state of the art four wheel disks with ABS etc. Now we look at my truck...at 144,000 lbs I have 16 big old drum brakes that havent changed much in technology since the turn of the 20th century each one of those brakes is responsible for stopping 9000 lbs ( no there isn't a decimal in there). The amount of heat generated durning a stop like that is unbelivable and chances are that after doing a stop like that something would be on fire... betcha wanna be a truck driver now eh? not!
The same thing goes for trains exept that trains have less axles per trailer than a truck hence less brakes per pound. Something else to consider with both trucks and trains is that they use air brakes... Air takes time to travel from one end of a vehicle to the other in a truck about 1/2 a second from my foot to the rear axle on my trailer, my truck is 'only' 75 ft long, some of the trains that I've seen are close to a mile long with a little math you could figure out that it would take a long time for the air pressure to reach the last car on a train hense a fairly long time to reach maximum braking power.

Answer 7

I was a train driver and take it from me, they are heavy blighters!!!

You try stopping an 8 car (2 locomotive + 6 bogey) hulk of metal travelling at 60MPH with 1500 passengers on board. And those brakes are big rasping metal beasts but it still takes an age to stop. Its strange really that in most trains these days the cabin which people get into is free floating above the axles... I remember a train hitting the buffers at Tattenham Corner many years ago... the lower part of the train stopped just through the buffers but the cabin slid off and continued through the ticket hall!!!

In summary, they are big, heavy and have plenty of motion behind them. Its a wonder they stop at all!!!

Answer 8

All above answers are "partially correct". The train has a lot of mass with limited traction (steel on steel rather than rubber on concrete) but the REAL reason is that the brakes don't come on all at once. Lines fill with pressure and the whole line (as long as the train) has to fill to get the brakes on. There is new research for individual valves at each car with a signal operating the brakes but it is still in research. You could look at other sites but I attached one here.

Answer 9

Because they got bald steel tyres running on steel rails, these trains actually stop pretty well 125 to 0 in 1.25 miles even if uk HST s are nearer 700 feet long than 8000.
There is a huge amount of nonsense above about slow acting brakes, the old railway companies used slow acting brakes pre war , 1st War Great Western, 2nd the others, and Mallard was engaged in brake tests testing much the same brakes city of Truro had in 1904, newly introduced on to the LNER , when it achieved the world speed record of 126 MPH on 3rd Juy 1938.
HSTs and MK 3 and 4 coaches use disc brakes but it is the lack of adhesion between rail and wheel which limits braking.

Answer 10

The momentum of the train and the lack of adhesion between the wheels and the rails means there is an optimum breaking force that can be applied before the wheels will lock. The weight of the train is so great that it requires such great distances to stop hence why on the railway system you have advanced signals to warn the driver to stop.