Freight versus Passenger Trains - different braking....

[Alastair Carr]

I know this is an odd question, but why do freight trains and passenger trains have different braking distances......?

They should be the same: the coeffecient of friction..... Is this true, or does the coefficient of friction change, say with temperature?

Can someone explain this to me in terms of physics? Obviously it depends on things like leaves on the line and gradient, but lets assume they're on a level track with the same rails and wheels.....

Thanks, I'd really appreciate it....

 

[GB]

Freight trains are a lot heavier and due to their length it takes longer for brakes to apply along the whole length of the train.

 

[matchmaker]

Freight trains are a lot heavier and due to their length it takes longer for brakes to apply along the whole length of the train.

Yes. 2300 tonnes of loaded coal train will take a lot more stopping than a few hundred tonnes of passenger train.

 

[Jamesb1974]

Different kinds of brakes as well. Most if not all passenger stock have disc brakes, with either the brake disc being part of the wheel centre or mounted inboard, on the axle. A larger surface area to dissipate the heat from braking (brakes convert motion to heat) means stronger braking.

Most if not all freight stock have traditional 'tread brakes'. Cast iron or composite brake shoes/blocks pressing against the tread of the wheel (ie the bit that actually rides on the rail). Therefore a smaller surface area to brake with. I think wagon brakes have remained 'traditional' for ease/cost of regular maintenance, but I could be wrong.

But I can personally attest that different wagons have better brakes than others. IPA car carriers and 'Cargowaggon' ferry vans only need a glance at the auto brake to stop you, usually yards from where you wanted to stop. Other wagons require more brake application to bring your speed down.

 

[cossie4i]

A lot of 2 axle wagons also only brake on 2 wheels, not all 4.

After driving passenger trains for the last 23years and now a freight driver the braking is vastly different. Lot more skill required driving freight.

 

[edwin_m]

The deceleration rate of the train, as a proportion of the acceleration of gravity, must always be less than the coefficient of friction otherwise the wheels will lock up. This would probably extend the stopping distances and the friction heating from the sliding also creates "wheelflats" in the wheel tread, that have to be removed with a lathe. So the brakes are designed so that the maximum deceleration they produce will be less than the coefficient of friction in normal conditions.

The coefficient of friction can become a lot less on a damp rail, or one that carries residue from leaf fall or oil from leaks on trains. Then the driver has to use a lower rate of braking to avoid locking the wheels, although most trains now have sand dispensers that can increase the available friction.

As stated above most passenger trains have disc brakes, which also also include wheelslide protection systems. WSP is like ABS on a car, and if working properly can give better deceleration in slippery conditions without causing wheelflats. Freight trains have much simpler brakes that can't give the same stopping distances.

 

[ac6000cw]

Can someone explain this to me in terms of physics?

In simple physics terms, the mass of the train and it's velocity mean it has kinetic energy - Kinetic energy = 0.5 * mass * square of the velocity

If using conventional friction brakes only, if braking to a stop all of that kinetic energy has to be transformed into heat.

Because of practical limits to the amount of heat that can be absorbed (and dissipated to the air) by the wheels/brake discs/brake shoes, the considerably greater mass of a fully loaded freight train compared to a passenger train per axle will result in longer braking distances (from the same speed, and other factors being equal). To put it another way, if the maximum allowable energy dissipation per second per axle is the same, the freight train needs more time to convert it's greater kinetic energy into heat.

As others have said, there are all sorts of practical engineering variables and differences in braking systems which come into play in the real world to affect braking performance - the above is a simple 'perfect world', fundamental physics explanation for part of the differences in stopping distances.

A lot of physics 'problems' can be easily analysed by looking at where the energy goes and how it is transformed, since it 'can be neither created nor be destroyed'

 

[HSTEd]

Since we have not adopted ECP brakes on freight trains, the air brake system will only apply at a rate of roughly 200m/s.
So on a 750m train you could have 3-4 seconds before the brakes at the rear of the train start to apply.
This can be significant at 70mph.

ECP brakes also have other benefits and can drastically reduce braking distances.
A 20,000t Taconite train in the US that was fitted with ECP for trials can stop from 38mph in just 560m. A train 1830m long stops in 560m.

 

[Jamesb1974]

So on a 750m train you could have 3-4 seconds before the brakes at the rear of the train start to apply.
This can be significant at 70mph.

One important point overlooked is the amount of time it takes for the brakes to release after an application. The brakes take longer to release than they do to apply. Freight locomotives have an 'airflow meter' which measures the airflow (no sh*t!) in the train pipe and shows the driver how fast the brakes are releasing. A short train will show a fairly rapid movement of the airflow meter back to its static position (not necessarily zero on the gauge), while a longer train will take more time for the needle to fall back to its static position.

Unlike a car, you don't apply the brake until the desired speed is met. For example, lets say you need to brake from 60mph to 30mph with a 2500ft long train. You would need to time the application and release inputs so that the brakes are releasing as you reach your desired speed. If you tried to brake as you would do in a car, ie keep the brake applied until you reached 30mph then release, you'd find yourself either coming to a grinding halt or having to reapply power to bring your speed back up to 30mph.

 

[ac6000cw]

Since we have not adopted ECP brakes on freight trains, the air brake system will only apply at a rate of roughly 200m/s.
So on a 750m train you could have 3-4 seconds before the brakes at the rear of the train start to apply.
This can be significant at 70mph.

ECP brakes also have other benefits and can drastically reduce braking distances.
A 20,000t Taconite train in the US that was fitted with ECP for trials can stop from 38mph in just 560m. A train 1830m long stops in 560m.

Just to clarify, ECP = Electronically Controlled Pneumatic brakes. This is essentially the freight train version of what has become the standard method of brake control on modern multiple-unit passenger trains.

 

[Alastair Carr]

OK,

For those of you who've stated that tonnage is a factor, I would very much like to show you what I've been taught in physics.


It is:

c=Ff/N

where c is the coefficient of friction, Ff is the friction force and N is the normal reaction.

This means that however heavy you are, the stopping distance will be the same, because the more Normal reaction (which is equal to the weight) the higher the friction force....

Please, people tell me I'm right, about this, aren't I?

 

[edwin_m]

In simple physics terms, the mass of the train and it's velocity mean it has kinetic energy - Kinetic energy = 0.5 * mass * square of the velocity

If using conventional friction brakes only, if braking to a stop all of that kinetic energy has to be transformed into heat.

Because of practical limits to the amount of heat that can be absorbed (and dissipated to the air) by the wheels/brake discs/brake shoes, the considerably greater mass of a fully loaded freight train compared to a passenger train per axle will result in longer braking distances (from the same speed, and other factors being equal). To put it another way, if the maximum allowable energy dissipation per second per axle is the same, the freight train needs more time to convert it's greater kinetic energy into heat.

As others have said, there are all sorts of practical engineering variables and differences in braking systems which come into play in the real world to affect braking performance - the above is a simple 'perfect world', fundamental physics explanation for part of the differences in stopping distances.

A lot of physics 'problems' can be easily analysed by looking at where the energy goes and how it is transformed, since it 'can be neither created nor be destroyed'

There is another factor to consider when thinking about the energy dissipation. This is that the rate of converting kinetic energy to heat is speed dependent. Work equals force times distance, so power equals force times speed. The force is related to the deceleration rate which is constant over the speed range for most brakes, so a brake at 100mph will be dealing with twice as much heat creation as one at 50mph.

Fortunately at higher speeds there is more airflow to cool the brakes.
--- old post above --- --- new post below ---

OK,

For those of you who've stated that tonnage is a factor, I would very much like to show you what I've been taught in physics.


It is:

c=Ff/N

where c is the coefficient of friction, Ff is the friction force and N is the normal reaction.

This means that however heavy you are, the stopping distance will be the same, because the more Normal reaction (which is equal to the weight) the higher the friction force....

Please, people tell me I'm right, about this, aren't I?

You are correct in principle. However there are various real-life factors which do make some difference to stopping distance, as put forward in posts above. They don't relate directly to the weight of the train but some of them are indirectly affected, principally propagation times through the air pipe of longer trains, and issues with being able to dissipate heat from the brake equipment.

 

[DaleCooper]

There is another factor to consider when thinking about the energy dissipation. This is that the rate of converting kinetic energy to heat is speed dependent. Work equals force times distance, so power equals force times speed. The force is related to the deceleration rate which is constant over the speed range for most brakes, so a brake at 100mph will be dealing with twice as much heat creation as one at 50mph.

Kinetic energy is proportional to the square of the velocity so at 100mph there would be four times as much energy (heat) to be dissipated as at 50mph.

 

[westcoaster]

I know this is an odd question, but why do freight trains and passenger trains have different braking distances......?

They should be the same: the coeffecient of friction..... Is this true, or does the coefficient of friction change, say with temperature?

Can someone explain this to me in terms of physics? Obviously it depends on things like leaves on the line and gradient, but lets assume they're on a level track with the same rails and wheels.....

Thanks, I'd really appreciate it....

Panther thing to factor in is most new rolling stock (passenger) also use rheostatic brakes basically putting the motors in reverse to slow down, and minimal air braking, and the final 7mph and below uses air braking only.
--- old post above --- --- new post below ---

I know this is an odd question, but why do freight trains and passenger trains have different braking distances......?

They should be the same: the coeffecient of friction..... Is this true, or does the coefficient of friction change, say with temperature?

Can someone explain this to me in terms of physics? Obviously it depends on things like leaves on the line and gradient, but lets assume they're on a level track with the same rails and wheels.....

Thanks, I'd really appreciate it....

Another thing to factor in is most new rolling stock (passenger) also use rheostatic brakes basically putting the motors in reverse to slow down, and minimal air braking, and the final 7mph and below uses air braking only.

 

[ac6000cw]

There is another factor to consider when thinking about the energy dissipation. This is that the rate of converting kinetic energy to heat is speed dependent. Work equals force times distance, so power equals force times speed. The force is related to the deceleration rate which is constant over the speed range for most brakes, so a brake at 100mph will be dealing with twice as much heat creation as one at 50mph.

Fortunately at higher speeds there is more airflow to cool the brakes.

...and there is four times as much kinetic energy to dissipate overall (since it is proportional to the square of the speed).

This is something that those of us old enough to ride on HSTs when they were first introduced were reminded of every time they braked from high speed - the smell of very hot brake pads which pervaded the train was almost as characteristic as the scream of a Valenta turbocharger . Stopping an HST from 125mph in the same distance as a train of equivalent weight from 100mph means dissipating 56% more energy in the brakes (and over a shorter time, so the power difference is even higher). Maintaining the same stopping distance as the 100mph trains they replaced (so the signals didn't have to be moved) was a basic HST design requirement.

 

[AM9]

With passenger vehicles, the difference between tare and gross weight is less than with freight vehicles. A typical coach weighs 40T and a full load of passengers (120 maybe ) would be say 120 x 80kg = 9.6T, i.e. a 24% increase in weight. A freight wagon could have a 15T tare weight and carry a 40T load which would be a 267% increase in weight.
Now brakes would need to cope with the maximum vehicle weight, so how would they be applied if the wagon was unloaded in a train with many fully loaded wagons? Without the load, the lack of available weight on the wheels would cause slippage.

 

[HSTEd]

This means that however heavy you are, the stopping distance will be the same, because the more Normal reaction (which is equal to the weight) the higher the friction force....

Please, people tell me I'm right, about this, aren't I?

In theory this would be true, however the mechanism that is used to alter the braking force applied by the air brake based upon the weight of the vehicle is rather crude.
This means that a fully loaded vehicle is actually only capable of a reduced deceleration compared to an empty one.
--- old post above --- --- new post below ---

With passenger vehicles, the difference between tare and gross weight is less than with freight vehicles. A typical coach weighs 40T and a full load of passengers (120 maybe ) would be say 120 x 80kg = 9.6T, i.e. a 24% increase in weight. A freight wagon could have a 15T tare weight and carry a 40T load which would be a 267% increase in weight.
Now brakes would need to cope with the maximum vehicle weight, so how would they be applied if the wagon was unloaded in a train with many fully loaded wagons? Without the load, the lack of available weight on the wheels would cause slippage.

There is a mechanism in the suspension of the vehicle that effectively measures the weight of the vehicle based on the length of the suspension springs - this is then used to alter the amount of force applied by the air brake.
It is however very crude, although far better than what happened before.

 

[edwin_m]

Kinetic energy is proportional to the square of the velocity so at 100mph there would be four times as much energy (heat) to be dissipated as at 50mph.

Yes, but my point was the rate of conversion of that energy to heat is only directly proportional to speed. It may be at bit tricky to reconcile these two facts, but it might help to remember that the time taken to stop is also proportional to the speed.

Brake systems need to cope with dissipating the total energy of the train during a stop, but they also need to cope with the higher rate of generation of that energy during the first part of the deceleration.

 

[HSTEd]

But in summary, technology is available that would allow freight trains to operate with the same braking distances as passenger trains - but in the freight railway technological progress always moves as a glacial pace.

 

[edwin_m]

This is something that those of us old enough to ride on HSTs when they were first introduced were reminded of every time they braked from high speed - the smell of very hot brake pads which pervaded the train was almost as characteristic as the scream of a Valenta turbocharger . Stopping an HST from 125mph in the same distance as a train of equivalent weight from 100mph means dissipating 56% more energy in the brakes (and over a shorter time, so the power difference is even higher). Maintaining the same stopping distance as the 100mph trains they replaced (so the signals didn't have to be moved) was a basic HST design requirement.

Oddly enough I smelt that smell on the Midland Main Line last week - for the first time in many years and it really took me back. So it must still happen if the brake application is unusually severe.

The HST is an exception to the rule that deceleration rates are constant across the speed range - it has a lower rate of deceleration above 100mph. I think this is due to being unable to dissipate the heat quickly enough at the top end of the speed range.

 

[Jamesb1974]

There is a mechanism in the suspension of the vehicle that effectively measures the weight of the vehicle based on the length of the suspension springs - this is then used to alter the amount of force applied by the air brake.
It is however very crude, although far better than what happened before.

A proportional load valve or load sensing valve. Some wagons also have an empty/loaded changeover lever which is a manual version of the above. I think they come in two types. Either the type with an arm that measures the wagon body dropping on the springs (as stated above) or a diaphragm type.

 

[Mr. Neutron]

Guess it's all about weight

 

[ac6000cw]

22 posts and no one has mentioned the role that locomotive dynamic (rheostatic) braking could play in reducing freight braking distances, if only the 66's were equipped with it....(the 68's and 70's have it).

EMD offered it as an option on their first mainline freight diesels in 1941, and it has transformed the way trains are handled in hilly terrain since then (and saved a fortune in brake shoe replacement )

 

[HSTEd]

Ironically moving to Electronically controlled brakes tends to increase brake shoe wear because train crews start making more use of friction brakes as they have far better stopping performance than a rheostatic brake that can only use the locomotive's adhesion.

 

[Jamesb1974]

22 posts and no one has mentioned the role that locomotive dynamic (rheostatic) braking could play in reducing freight braking distances, if only the 66's were equipped with it....(the 68's and 70's have it).

EMD offered it as an option on their first mainline freight diesels in 1941, and it has transformed the way trains are handled in hilly terrain since then (and saved a fortune in brake shoe replacement )

But surely dynamic braking only applies to the locomotive and not the train?

 

[DownSouth]

But surely dynamic braking only applies to the locomotive and not the train?

The braking effort transfers to the rest of the train through the couplers.

It works with certain electric locos in Britain, until the more recent development of more compactly packaged prime movers it was not possible with British diesel locos due to the My First Train loading gauge and the preference for two cabs on every loco.

 

[edwin_m]

If I recall correctly class 50s had dynamic braking when new back in the 60s, though I think it was removed on refurbishment along with a number of other features that made them too complicated to maintain.

Dynamic braking is particularly useful for slowing a train on long descents, because of the risk of running out of air when making prolonged light applications of a single-pipe air brake. The loco braking and adhesion should be sufficient to keep the train under control on these gradients, otherwise the same loco wouldn't be able to haul the same train in the other direction! I guess in a loco without dynamic braking, the drivers have to use the loco brake on its own in this situation.

 

[AndyNLondon]

OK,

For those of you who've stated that tonnage is a factor, I would very much like to show you what I've been taught in physics.


It is:

c=Ff/N

where c is the coefficient of friction, Ff is the friction force and N is the normal reaction.

This means that however heavy you are, the stopping distance will be the same, because the more Normal reaction (which is equal to the weight) the higher the friction force....

Please, people tell me I'm right, about this, aren't I?

I don't have any experience of how train brakes actually work, so with that in mind...
The equation you mention there is for sliding friction, i.e. the brake blocks on discs/wheels. So, the normal force in that context isn't the weight, it's the brake force pressing the blocks onto the disc/wheel, and the friction force is slowing the rotation of the wheels. So, assuming otherwise-identical trains with the same brake force, the heavier one will decelerate less rapidly (F=ma, same force & larger mass implies smaller acceleration) and therefore take longer to stop.

 

[HSTEd]

However the limiting factor on how fast a train can decelerate is the amount of force that can be applied at the rail-wheel interface without the train starting to slide.