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Emergency stopping brakes

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AndrewE

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edwin_m

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Old trams in London had no auxiliary power systems on the vehicles, such as hydraulics or air for brakes (nor even a 12 volt low voltage supply), and so everything had to be done, if at all, at the full 550v line voltage. The controller had typically about 8 forward power notches, four series and then four parallel. Back past the 0 point in the opposite direction were braking notches (rheostatic brake), maybe two, which operated by essentially reversing the motor connections to make them generators, producing a current instead of using it, which was then dissipated in the normal power resistances. Beyond this, further notches caused the magnetic brake (if fitted) to come into play as well, clamping the magnetic shoes to the rail.

The electric rheostatic brake falls off as speed does, so for a final stop another brake is required. The mag brake has the opposite characteristic, and is more effective at slower speeds, so the two together are complementary. Note that none of this has used conventional brake shoes on the wheels. All trams had these as well, applied by a large wheel or handle.

London went extensively for the magnetic brake, Glasgow was more modern and installed air compressors and used air brakes instead on brake shoes, even fitting these to older vehicles. Manchester apparently had neither, and until the end of trams there in 1949 pulled up at each stop by the driver hauling on the handbrake.

Electric brakes are fine but depend on the line voltage being available, so if descending a hill and the power supply fails (or, for trams, the trolley pole comes off), the brake is unavailable, and therefore you do need full braking ability by another means.

In some cases the power regenerated by electric braking in the motors was used to energise a magnetic brake. How this works, and what went wrong on one occasion, is described in the accident report into a runaway in Bournemouth in 1908.

http://www.royhodges.co.uk/Bournemouth Tram Crash 1908.pdf
 

AndrewE

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In some cases the power regenerated by electric braking in the motors was used to energise a magnetic brake. How this works, and what went wrong on one occasion, is described in the accident report into a runaway in Bournemouth in 1908.

http://www.royhodges.co.uk/Bournemouth Tram Crash 1908.pdf

That seems to say that the controller was faulty. I would have thought that the fundamental problem is that the power for rheostatic / regenerative braking evaporates as you slow down. It has been pointed out in the Italian crash thread that kinetic energy (i.e. available to drive the braking) is proportional to the square of the speed. Hence the switch to friction or other brakes as speed drops.
I did notice a solid clunk on the Metrolink tram around Oldham each time we came to a stand on a significant gradient. I suspected it might be an electromagnetic track brake being applied.
 
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AndrewE

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If we are de-staffing the railway then the technology has to work all the time.
The mother of one of two teenage girls killed at a level crossing has expressed her anger after discovering a safety feature installed in the wake of the tragedy was not working just days before the 10th anniversary of her daughter's death.
Tina Hughes, 57, found that an automatic locking system was not working when she visited the site last week where her daughter was killed in Elsenham, Essex .
Tina Hughes, the mother of Olivia Bazlinton, at London Bridge station ahead of the 10th anniversary of her daughter's death Charlotte Thompson, 13, and Olivia Bazlinton, 14, were killed after... read more
from http://www.briefreport.co.uk/news/m...-down-ahead-of-death-anniversary-3682098.html
There have been persistent complaints that the communications or indications at occupation crossings aren't maintained in working order. Culminating in a fatality in Scotland, I think.
 

Philip Phlopp

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If we are de-staffing the railway then the technology has to work all the time.
from http://www.briefreport.co.uk/news/m...-down-ahead-of-death-anniversary-3682098.html
There have been persistent complaints that the communications or indications at occupation crossings aren't maintained in working order. Culminating in a fatality in Scotland, I think.

I'm not talking about crossing technology, I'm talking about closing crossings completely.

You'll never control users, attendants have a hard enough time of it today, plenty of instances where crossing attendants have been hit by passing vehicles, assaulted or abused, where gates are being blocked and vandalised or users are so uncooperative, the crossing is a major source of delay minutes.

The un-manned, user worked crossings can be fully worse - frequently vandalised, left open, signalling disregarded and users who chance their luck, often because they know the passenger train schedule and come unstuck with a NR Test Train, RHTT or freight.

Technology doesn't fix any of that, other than the odd deterrent that CCTV will make a prosecution easier. Bridges, underpasses, and general closures are needed, and are being pursued.
 

edwin_m

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I did notice a solid clunk on the Metrolink tram around Oldham each time we came to a stand on a significant gradient. I suspected it might be an electromagnetic track brake being applied.

Like all UK light rail networks, Metrolink doesn't and shouldn't use the track brake in normal service. The drive on sight principle means that they should always be able to stop short of any obstruction using the service brake, with the track brake available as a last resort for situations the driver couldn't foresee, such as a cyclist unexpectedly veering into the path of a tram. With a deceleration of about 10%g, the service brake (supported by automatic sanding) is well able to stop a tram on the worst gradient of about 6%, though the stopping distances will be much longer than on the level and the descent from the heights of Oldham will therefore be pretty cautious.

I believe the clunk is the application/release of the parking brake. With AC motors dynamic braking is available almost down to a stop but the spring-applied parking brake is able to hold a tram still on the heaviest gradient. In the event of failure of the dynamic brakes the friction brakes are automatically applied instead and can stop the tram in a similar distance.
 

XDM

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I believe some German trains use eddy current brakes,which are independent of friction between rail & wheel & so ideal for emergency stop. Network rail have rejected its use,apparently. They argue that in service use it always heats up the same section of rail where trains normally brake,with unknown effect on track,a fair objection. But the system is very light & very effective & would be ideal in a rare emergency or an inevitable buffer stop collision where there is a loss of grip due to poor rail condition. could we abandon defensive driving if all trains had an emergency eddy current brake? Is the above info about eddy current brakes correct,which is just from chitchat with engineer colleagues?
 

notadriver

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Is it true in Japan and Germany trains approach and brake very hard for stations ? Do we drive too defensively ?


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ComUtoR

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Do we drive too defensively ?

Every time I've had to put the train into emergency or my train has come to a stand after a slip I am very glad I drive as defensive as I do.

We have many Drivers who brake relatively late and are still within the defensive driving policy.

Personally I don't think you can drive too defensively.
 

Domh245

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I believe some German trains use eddy current brakes,which are independent of friction between rail & wheel & so ideal for emergency stop. Network rail have rejected its use,apparently. They argue that in service use it always heats up the same section of rail where trains normally brake,with unknown effect on track,a fair objection. But the system is very light & very effective & would be ideal in a rare emergency or an inevitable buffer stop collision where there is a loss of grip due to poor rail condition. could we abandon defensive driving if all trains had an emergency eddy current brake? Is the above info about eddy current brakes correct,which is just from chitchat with engineer colleagues?

It would seem that you are correct, ICE 3 units are fitted with Linear Eddy brakes. It is possible to do rotary Eddy Brakes (indeed it is done on 700 series Shinkansen) but that still relies on the wheel/rail interface. I can see why NR object to linear eddy brakes for heating reasons, although perhaps if they only use it in winter they can cut down on point heaters :P

As for abandoning defensive driving, I would suggest that would be a daft idea. As it is, trains are capable of much better performance then they exhibit, the idea of defensive driving is to prevent accidents from pushing the limit too much. You might be able to revise defensive driving policies, if you don't mind causing injury to passengers when a driver makes an emergency brake application and sends everyone flying.
 

MisterT

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Do I understand correctly from this thread that main line trains in the UK don't have any magnetic track brakes?
Almost all passenger trains in the Netherlands have them, except for the Loco hauled 'intercity direct' service and the oldest SGM-2 sprinter trains (from which the latter is not allowed to run on its own during the fall season, because of the lack of magnetic track brakes and need to be coupled with a SGM-3 sprinter train during this season).
Eddy current brakes (or "wervelstroomrem" in Dutch) are not allowed in the Netherlands, because we use track circuit train detection on most of the tracks.
We only use the magnetic track brakes in case of an emergency, and some trains (VIRM double deck EMUs) use them as parking brake, as the VIRM has permanent magnets, instead of the electromagnets the other trains have.
 

Clip

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I do have a concern about passengers being thrown about nowadays, more from the current desire to reduce seating and make more passengers stand. Standing passengers are not allowed on coaches on motorways but we somehow accept them on trains at comparable speeds.

Time and again this gets mentioned and time and again Ill point out thata people standing up wedged in will cause less injuries than people sat down who will be flung about the carriage. The ban on coaches is due to the nature of where coaches operate and theres other users which can cause an accident which isnt the fault of the coach thus increasing the likelyhood of people being injured in a coach.
 

edwin_m

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The new 399 tram-train would be though?

Yes, and as mentioned above the Tyne and Wear Metrocars that run to Sunderland on Network Rail infrastructure. I'm not sure however whether either is allowed to use the magnetic brake on the main line.
--- old post above --- --- new post below ---
But the system is very light & very effective & would be ideal in a rare emergency or an inevitable buffer stop collision where there is a loss of grip due to poor rail condition.

The braking force produced by an eddy current brake is speed-dependent, so it wouldn't be much use in averting a low-speed collision such as a buffer stop.
 

bangor-toad

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Last year a company called Mole Solutions Ltd won a RSSB competition to develop a prototype linear induction motor that would be mounted between the tracks and could / would be used as an emergency brake.

It would act on the magnetic material in the train itself and provide the required forces.
The physics of it are sound but I have no idea if the engineering issues turned out to be solvable at a sensible cost.

Does anyone know if this project worked as hoped / expected?
Cheers,
Mr Toad
 

Taunton

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The "Magnetic Brake", old term, is actually no more magnetic than an electric motor, it just uses some of the same elements in a different manner. The blocks are just depressed onto the rail head by a force comparable to that from a motor which turns the wheels.

Things that actually apply a current of some sort to the rails, such as the Eddy Current discussed above, would likely interfere with track circuits and therefore not be acceptable.
 

FordFocus

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Brakes of the current design can not be much improved. Applying a greater force to the wheels would simply result in them sliding on the track, as already happens under some conditions.

Magnetic track brakes are a possibility, but have found little favour for main line stock.

A much quicker stop would also be more dangerous to passengers who could be thrown about and even killed if unlucky.

Also, callous though this sounds, I consider that the lives of the train driver and the paying passengers are worth more than those of a trespasser or suicide.
At present a train driver would apply emergency braking on seeing a trespasser or potential suicide. Depending on speeds and distances, the train may or may not stop in time.
The risks to the driver and passengers are minimal, the risk to the trespasser is significant. But remember that the trespasser CHOSE TO TRESPASS and must accept the perhaps fatal consequences.

Now consider a train with greatly improved brakes as has been proposed. On seeing the trespasser, the driver would apply the much improved brake and thereby risk serious injury or even death to any unlucky passengers.
The trespasser or potential suicide would likely survive to try again.

Is it worth risking the lives of innocent customers, in order to protect the lives of those who are knowingly risking their own lives by deliberate action or by wanton careless use of crossings.

Accidents involving a train colliding with another train at significant speed are now extremely rare. In general such an accident usually requires a most serious and very rare lapse by the driver, such as passing a signal at danger, AND AT THE SAME TIME a failure of TPWS or other systems that are installed to protect against such mistakes.

A very good post.

It reminds me of a story about a driver who saw a trespasser but put the brake into full service instead of a slightly enhanced emergency brake thinking the person wasn't in a position to kill themselves but was using the railway as a short cut. In the end it was a suicidal person but the aftermath of having to explain it to the police the difference between braking steps.

If the mainline railway developed and adapted some form of enhanced emergency brake that has a risk of injuring passengers from de acceleration forces then it would have to have some serious thought into it's use. From a drivers point of view, it could be another way of criticising afterwards actions during a split second decision.
 

AndrewE

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The "Magnetic Brake", old term, is actually no more magnetic than an electric motor, it just uses some of the same elements in a different manner. The blocks are just depressed onto the rail head by a force comparable to that from a motor which turns the wheels

Perhaps you had better explain that to Knorr-Bremse, and update Wikipeia while you are at it.
http://www.knorr-bremse.com/media/d...ke_systems/Magnetic_Track_Brake_P_1269_EN.pdf
says
"Magnetic track brakes are magnetically attracted to the
rails. The braking force is built up by using the friction
between the magnetic track brake and rail. Automatic
braking and also emergency braking are typical tasks for
magnetic track brakes

The magnetic track brake is always unregulated and
applies its maximum braking force"
so as I thought, they dump the OLE through a big electro-magnet that sticks to the track.
 

edwin_m

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The "Magnetic Brake", old term, is actually no more magnetic than an electric motor, it just uses some of the same elements in a different manner. The blocks are just depressed onto the rail head by a force comparable to that from a motor which turns the wheels.

Perhaps you had better explain that to Knorr-Bremse, and update Wikipeia while you are at it.
http://www.knorr-bremse.com/media/d...ke_systems/Magnetic_Track_Brake_P_1269_EN.pdf
says
so as I thought, they dump the OLE through a big electro-magnet that sticks to the track.

You're both right. Magnetic brakes (whether friction or eddy current) and electric motors both contain electromagnets formed from coils of wire.
 

AndrewE

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You're both right. Magnetic brakes (whether friction or eddy current) and electric motors both contain electromagnets formed from coils of wire.

Surely a motor is designed to maintain a distance between the moving parts (rotor and stators) which implies power losses, while a track brake can exert the maximum possible force because the gap is zero - I would expect the attraction to be inversely proportional to the square of the distance or something similar. A lot more magnetic than an electric motor.
And if you put OLE power through the magnet it will be a lot more effective than rheostatic braking at low speeds, although I take the point that ac motors and synchronous drives are very clever...
 

edwin_m

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Surely a motor is designed to maintain a distance between the moving parts (rotor and stators) which implies power losses, while a track brake can exert the maximum possible force because the gap is zero - I would expect the attraction to be inversely proportional to the square of the distance or something similar. A lot more magnetic than an electric motor.

There are numerous other factors coming into play here so it's nowhere near as simple.

For a start the magnets aren't actually doing any work because there is no movement in the direction of the magnetic force, so the only current they are using is whatever is necessary to obtain magnetisation. A permanent magnet would work just as well and be infinitely efficient if someone could think of a way of getting and keeping it off the rail when not in use.

And if you put OLE power through the magnet it will be a lot more effective than rheostatic braking at low speeds, although I take the point that ac motors and synchronous drives are very clever...

But if you use regenerative braking you are getting power back from the motor but not from the magnetic track brake. And AC motors do allow regenerative braking almost down to 0mph. You basically reverse the +/- sign on a parameter or two in the software that accelerates the train, so the braking rate is about the same as the acceleration rate.

A couple more problems with magnetic track brakes: [deleted], and it doesn't work on some of the non-magnetic alloys used in some specialist crossing castings. I imagine if a train went over one of those while using the magnetic brake, you'd get a series of lurches as each bogie lost and regained brake force.
 
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AndrewE

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There are numerous other factors coming into play here so it's nowhere near as simple.

For a start the magnets aren't actually doing any work because there is no movement in the direction of the magnetic force, so the only current they are using is whatever is necessary to obtain magnetisation. A permanent magnet would work just as well and be infinitely efficient if someone could think of a way of getting and keeping it off the rail when not in use.
So that's a pretty irrelevant attempt at a put-down than. The idea is to apply extra friction in an emergency, not to let the magnet do any other work.

But if you use regenerative braking you are getting power back from the motor but not from the magnetic track brake. And AC motors do allow regenerative braking almost down to 0mph. You basically reverse the +/- sign on a parameter or two in the software that accelerates the train, so the braking rate is about the same as the acceleration rate.
This is about emergency braking, and regeneration (or dumping the current elsewhere) is irrelevant.

A couple more problems with magnetic track brakes: the magnetic force effectively increases the weight of the train when it is in use,
I think not, the magnet hangs on springs and when energised sticks itself to the track and opposes the train's momentum by working against a big bracket bolted to the bogie frames.
and it doesn't work on some of the non-magnetic alloys used in some specialist crossing castings. I imagine if a train went over one of those while using the magnetic brake, you'd get a series of lurches as each bogie lost and regained brake force.
So if a train is crossing a fancy "non-magnetic alloy" (but basically steel) crossing in an emergency situation you think a series of jolts as each brake along the train temporarily loses (some of ) its effect would invalidate the principle?
 
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philthetube

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A very good post.

It reminds me of a story about a driver who saw a trespasser but put the brake into full service instead of a slightly enhanced emergency brake thinking the person wasn't in a position to kill themselves but was using the railway as a short cut. In the end it was a suicidal person but the aftermath of having to explain it to the police the difference between braking steps.

If the mainline railway developed and adapted some form of enhanced emergency brake that has a risk of injuring passengers from de acceleration forces then it would have to have some serious thought into it's use. From a drivers point of view, it could be another way of criticising afterwards actions during a split second decision.

If a driver has an extra decision to make, that will be an extra 2 seconds before a brake application is made. RAIB often talk about the decision time when drivers have an option.
 

edwin_m

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So that's a pretty irrelevant attempt at a put-down than.

Please don't put words into my mouth by inserting text within what you have quoted as being from me.

I'm not trying to put you down, but your previous statement comparing the magnetic fields in the motor and in the track brake is trying to compare apples and oranges, as these magnets are doing totally different things.

You are interested in getting the maximum retardation and not worried about efficiency - so let's take a real world example.

Trams decelerate at about 13%g on service braking and 25%g on hazard braking. So the electromagnetic track brakes are responsible for about 12%g of acceleration and the service brakes for a similar amount. You need both to obtain greater deceleration. Electromagnetic brakes on a train might also be less effective as it is heavier, and to have the same deceleration the magnetic force would need to increase in proportion to the weight.

On thinking further about it I was wrong to say use of the electromagnetic brake increases the effective weight of the train.
 

cjmillsnun

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Every time I've had to put the train into emergency or my train has come to a stand after a slip I am very glad I drive as defensive as I do.

We have many Drivers who brake relatively late and are still within the defensive driving policy.

Personally I don't think you can drive too defensively.

Agreed. The last thing we want is discouraging of defensive driving.
In a real emergency, the driver will make an emergency brake application, otherwise defensive driving reduces wear and prevents damage to the rails, reduces wear and prevents damage to the trains, and gives the passengers a smoother ride without compromising safety in any way.
 

dubscottie

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Talking as someone who has had the joys of being an a Luas tram in Dublin when its used the track brakes..

There is no way they could be used on mainline stock.. After the driver hit the anchors there must have been only 3-4 people that were either still standing or still in seats.. And this was at 30-40kph approaching a stop.

You could see the marks on the rails from where the brake hit them.
 
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