Why don't trains in Great Britain use eddy current brakes?

[rebmcr]

As I understand it, eddy current brakes have been routinely fitted to trains running in Germany since the 1990s. This technology uses an electromagnet to induce an electric field in the rails, the resistance of which produces braking force. Unlike the emergency electromagnet 'track brakes' fitted to street-running trams here, it does not touch the rails and no friction is involved, nor does using it cause any damage to infrastructure.

I don't believe it would be possible to regenerate current using such a system, but — similarly to the current norms that combine regenerative braking with a 'backup' conventional disc brake — it might be sensible to include the equipment as part of a suite of options that a train could use to slow down, or even held in reserve for when the wheels lose traction. Of course, the reason I thought of this is the recent collision near Salisbury, where it seems like a non-friction-based brake could have helped.

Is the absence of such technology down to a particular reason, or is it simply a case of "we just never started doing it"?

 

[Roast Veg]

Is the absence of such technology down to a particular reason, or is it simply a case of "we just never started doing it"?

How do the Germans mitigate against electromagnetic interference, out of curiosity? We have enough of that knocking out signals in 25kV OLE installations before we put some near track circuits.

 

[NSEFAN]

How does such a system work with track circuits? Would axle counters or special immunisation be needed to avoid interfering with the signalling?

 

[stuu]

As I understand it, eddy current brakes have been routinely fitted to trains running in Germany since the 1990s. This technology uses an electromagnet to induce an electric field in the rails, the resistance of which produces braking force. Unlike the emergency electromagnet 'track brakes' fitted to street-running trams here, it does not touch the rails and no friction is involved, nor does using it cause any damage to infrastructure.

I don't believe it would be possible to regenerate current using such a system, but — similarly to the current norms that combine regenerative braking with a 'backup' conventional disc brake — it might be sensible to include the equipment as part of a suite of options that a train could use to slow down, or even held in reserve for when the wheels lose traction. Of course, the reason I thought of this is the recent collision near Salisbury, where it seems like a non-friction-based brake could have helped.

Is the absence of such technology down to a particular reason, or is it simply a case of "we just never started doing it"?

They haven't been fitted routinely at all. They are fitted to ICE3 trains specifically for use on high speed lines where the infrastructure is designed for them and can't be used elsewhere

I would have thought magnetic track brakes would be better as an emergency option

 

[87015]

They haven't been fitted routinely at all. They are fitted to ICE3 trains specifically for use on high speed lines where the infrastructure is designed for them and can't be used elsewhere

I would have thought magnetic track brakes would be better as an emergency option

Lightweight DMUs have had what I've considered "magnetic" brakes going back to at least the 628s, but I believe they are normal use and not emergency only and certainly on regular infrastructure. I'm not sure if thats different to the above mentioned "eddy current" ones, but could be what is meant.

 

[Applepie356]

Is there a reason why magnetic track brakes haven’t been used on trains?

 

[Senex]

They haven't been fitted routinely at all. They are fitted to ICE3 trains specifically for use on high speed lines where the infrastructure is designed for them and can't be used elsewhere

I would have thought magnetic track brakes would be better as an emergency option

I thought DB had used magnetic track brakes (not eddy-current brakes) for a good many years. They certainly began experiments with them way back at the start of the 1930s, on a Fliegender Hamburger dmu. I can't find a good side-on close-up image of a Class 103, for example, but I'm pretty sure the magnetic brakes are easily visible between the wheels of the outer and middle axles in the Co[-Co] bogies.

 

[rebmcr]

Is there a reason why magnetic track brakes haven’t been used on trains?

They are really nasty to the rails, and to the underlying foundations. The only reason they're fitted to trams is because of the risk that pedestrians could step out at any moment without warning.

 

[Greybeard33]

They are really nasty to the rails, and to the underlying foundations. The only reason they're fitted to trams is because of the risk that pedestrians could step out at any moment without warning.

The magnetic track brakes on Manchester Metrolink and other light rail systems are used only for emergency braking, so track wear is really not an issue. But they are highly effective, giving retardation rates comparable to road vehicles.

 

[Irascible]

Specific to Salisbury, there might be a slight problem powering an electromagnet on a diesel-hydraulic multiple unit.

 

[edwin_m]

They are really nasty to the rails, and to the underlying foundations. The only reason they're fitted to trams is because of the risk that pedestrians could step out at any moment without warning.

Road vehicles could also unexpectedly get in the way.

The magnetic track brakes on Manchester Metrolink and other light rail systems are used only for emergency braking, so track wear is really not an issue. But they are highly effective, giving retardation rates comparable to road vehicles.

It is itself hazardous to anyone who happens to be standing up in the tram and not holding on to anything - as well as the higher retardation it applies more suddenly than a normal brake. So it should only be used to avoid a greater hazard such as a collision. Drivers are trained to anticipate such events and control their speed so as to be able to stop short of them using only the service brake. The track brake is then available strictly as a reserve to be used if somethin unexpected happens.

 

[Greybeard33]

Specific to Salisbury, there might be a slight problem powering an electromagnet on a diesel-hydraulic multiple unit.

I am sure that the current demand of the electromagnets would be well within the capability of the unit's batteries. Tram track brakes are not dependent on the OLE supply.

 

[Sir Felix Pole]

I can confirm that magnetic track brakes are highly effective - I was in the front seat of a Dijon (France) tram shortly after the system opened, when a car jumped the lights at an intersection as we were getting underway. The tram-driver reacted reacted remarked quickly and we stopped almost instantaneously - there was a loud thud as the brake clamped to the track. We missed the car by inches, but there were a few standees in the door wells who were thrown forward - fortunately no injuries. First generation trams on some systems had air operated track brakes instead, and indeed some hilly systems such as Stockport had hand-operated slipper brakes on the track as well.

 

[rebmcr]

I can confirm that magnetic track brakes are highly effective - I was in the front seat of a Dijon (France) tram shortly after the system opened, when a car jumped the lights at an intersection as we were getting underway. The tram-driver reacted reacted remarked quickly and we stopped almost instantaneously - there was a loud thud as the brake clamped to the track. We missed the car by inches, but there were a few standees in the door wells who were thrown forward - fortunately no injuries. First generation trams on some systems had air operated track brakes instead, and indeed some hilly systems such as Stockport had hand-operated slipper brakes on the track as well.

However, it would not be appropriate for every mainline driver to use them every time they experienced wheelslide.

When the SWR driver at Salisbury made the emergency brake demand, it's likely he could not yet see the potential collision with the GWR train, and so a system that can be used with more regularity would be useful instead — that's what I was asking about when starting this thread.

 

[HSTEd]

However, it would not be appropriate for every mainline driver to use them every time they experienced wheelslide.

When the SWR driver at Salisbury made the emergency brake demand, it's likely he could not yet see the potential collision with the GWR train, and so a system that can be used with more regularity would be useful instead — that's what I was asking about when starting this thread.

The interesting question would be how long elapsed between the driver realising a collision was imminent and the impact occuring?

You could imagine a controller have an emergency brake for the emergency brake, as it were.

Although I believe magnetic track brakes are relatively commonly used in some places - they do allow significant operational advantages in terms of stopping distances and such.

 

[rebmcr]

The interesting question would be how long elapsed between the driver realising a collision was imminent and the impact occuring?

12 seconds after using the service brake control, the Salisbury driver hit the emergency brake (probably before the red signal was even visible, and certainly before TPWS tried to do the same). It's a situation like that where the driver knows there's a problem and a risk, but does not yet know whether it will result in a SPAD/near miss/collision, where a non-destructive backup might be useful.

 

[driverd]

Absolutely don't know if this is the correct answer but I'll hazard a guess.

Cost.

Simply, it's never been done before. Given the nature of Britain's privatised network, investing in an expensive, new type of train brake, that may go the way of the APTs hydrokinetic, or have unforseen consequences, like that oh-so-revolutionary curved 385 windscreen, is likely to sit too high on the risk-reward scale. And that's not even considering the regulatory hoops introducing such a system would have to navigate before it would be accepted as suitably fail safe from the ORRs perspective.

Compared to the costs and simplicity of introducing tried and tested air brakes, I imagine it's a pretty simple conclusion thats reached.

Obviously all entirely conjecture on my behalf but I wouldn't be remotely surprised if its not too far from reality.

 

[edwin_m]

And that's not even considering the regulatory hoops introducing such a system would have to navigate before it would be accepted as suitably fail safe from the ORRs perspective.

It wouldn't have to be failsafe in the traditional sense. In fact if any failure caused the brake to apply that could be more hazardous than just making it inoperable, because of the risk of injuries within the train from the sudden deceleration.

The logic is that the brake is only used in rare circumstances and an accident only results if the brake doesn't work in one of those rare events. If that happens the outcome is just the same as if the brake isn't fitted. Assuming the brake usually works when demanded, there will still be a net safety benefit.

This does require some form of automatic or manual testing to detect "latent faults" when there is a problem but it doesn't reveal itself until someone tries to use it. There would probably also be pressure to take a train out of service, or at least not let it start its working day, if the brake wasn't working.

 

[Bletchleyite]

I thought DB had used magnetic track brakes (not eddy-current brakes) for a good many years. They certainly began experiments with them way back at the start of the 1930s, on a Fliegender Hamburger dmu. I can't find a good side-on close-up image of a Class 103, for example, but I'm pretty sure the magnetic brakes are easily visible between the wheels of the outer and middle axles in the Co[-Co] bogies.

Pretty sure they are, or were, a condition of running over 160km/h and so are seen clearly on the bogies of most DB hauled coaches.

 

[Poppysdad]

Magnetic track brakes are used on mainline trains in a number of other countries and have been considered at various times within the UK. There are a number of other technologies which have been tested which would improve the situation of poor adhesion, see link below

ADHERE 2020 Webinar Series (rssb.co.uk)

 

[bahnause]

Pretty sure they are, or were, a condition of running over 160km/h and so are seen clearly on the bogies of most DB hauled coaches.

They are necessary for speeds up to 160km/h with conventional lineside signalling with fixed braking distances. They are usually not necessary for speeds above 160km/h with in cab signalling systems. In this case the on board unit of the train will adjust the braking curve according to the available braking force.

Another thing to keep in mind is, that a magnetic brake on mainline trains usually turns of at speeds below 50km/h to keep deceleration within the permitted limits and to avoid a jolt when stopping.

 

[hexagon789]

They are necessary for speeds up to 160km/h with conventional lineside signalling with fixed braking distances. They are usually not necessary for speeds above 160km/h with in cab signalling systems. In this case the on board unit of the train will adjust the braking curve according to the available braking force.

When DB first introduced 200km/h running EM track brakes were the additional requirement against the requirements for 160km/h running. When they first went officially to 160 against the previous 140 post-war, it was disc brakes that were the additional requirement.

Pretty sure they are, or were, a condition of running over 160km/h and so are seen clearly on the bogies of most DB hauled coaches.

Yes, they were the additional requirement back in the 1960s when DB first introduced 200km/h running

 

[Jan]

At least these days though the common wisdom is that reaching the calculatory brake force percentage necessary for running at speeds > 140 km/h under conventional signalling is impossible without having either magnetic track brakes or eddy current brakes available (doesn't mean that they're actually used every time a train running at 160 km/h needs to brake for a red signal, but they need to be available for a worst case scenario, like e.g. a distant signal with minimum visibility and spaced at the minimum permissible distance, and in combination with a slight falling gradient leading to the train only just reaching the necessary brake force percentage at that particular location).

They haven't been fitted routinely at all. They are fitted to ICE3 trains specifically for use on high speed lines where the infrastructure is designed for them and can't be used elsewhere

Not quite – service braking is only allowed on slab track due to concerns about rail heating in a worst case scenario (hot summer day and lots of successive trains equipped with eddy current brakes following each other at close intervals and braking at the same spot), so in practice is indeed limited to the more recently built high speed lines (the first generation of high speed lines uses conventional ballasted track).

For emergency braking however rail heating is less of a concern, and conventional routes seeing regular ICE 3 traffic have correspondingly been adapted (immunisation of signalling and making sure that points motor covers and the like remain in place and don't start flying away after a brake application) such that eddy current brakes can be used during emergency braking (and even pure emergency brake usage is enough for allowing the eddy current brake to be counted against the required brake force percentage required for 160 km/h running under conventional signalling).

Because of all this fuss and kerfuffle, usage of the eddy current brake has indeed remained restricted to high speed trains, though, where the main benefit is probably seen as allowing higher braking forces at high speeds without placing undue demands on the friction brake (the dynamic brake is power-limited and can therefore only provided a lower brake force at higher speeds due to F = P / v). For anything else which "only" needs the > 140 km/h under conventional signalling capability, conventional magnetic track brakes remain the standard solution.

 

[edwin_m]

At least these days though the common wisdom is that reaching the calculatory brake force percentage necessary for running at speeds > 140 km/h under conventional signalling is impossible without having either magnetic track brakes or eddy current brakes available (doesn't mean that they're actually used every time a train running at 160 km/h needs to brake for a red signal, but they need to be available for a worst case scenario, like e.g. a distant signal with minimum visibility and spaced at the minimum permissible distance, and in combination with a slight falling gradient leading to the train only just reaching the necessary brake force percentage at that particular location).


Not quite – service braking is only allowed on slab track due to concerns about rail heating in a worst case scenario (hot summer day and lots of successive trains equipped with eddy current brakes following each other at close intervals and braking at the same spot), so in practice is indeed limited to the more recently built high speed lines (the first generation of high speed lines uses conventional ballasted track).

For emergency braking however rail heating is less of a concern, and conventional routes seeing regular ICE 3 traffic have correspondingly been adapted (immunisation of signalling and making sure that points motor covers and the like remain in place and don't start flying away after a brake application) such that eddy current brakes can be used during emergency braking (and even pure emergency brake usage is enough for allowing the eddy current brake to be counted against the required brake force percentage required for 160 km/h running under conventional signalling).

Because of all this fuss and kerfuffle, usage of the eddy current brake has indeed remained restricted to high speed trains, though, where the main benefit is probably seen as allowing higher braking forces at high speeds without placing undue demands on the friction brake (the dynamic brake is power-limited and can therefore only provided a lower brake force at higher speeds due to F = P / v). For anything else which "only" needs the > 140 km/h under conventional signalling capability, conventional magnetic track brakes remain the standard solution.

In the UK the signal spacing is set according to the line speed to ensure that adequate braking distance is available, exceptions being rare cases of extremely poor rail adhesion as appears to have happened in Salisbury last week. The UK adopted better braking (disc brakes) to increase from 160km/h to 200km/h without changing the signals, as mentioned somewhere above.

Your comments imply that the spacing from the distant to the stop signal is fixed and therefore increased braking is necessary at speeds above 140km/h. Is this something specific to the standards applying in Germany or some other country?

I'm also surprised that eddy current braking is needed for operation on any new high speed lines, because I don't believe they are mandated in the TSI so such a requirement is going against interoperability. The newer lines are also required to have ERTMS, and unlike conventional signalling this adjusts the spacing between trains to account for the braking distance of the train behind. So in theory this should safely separate trains regardless of the type of brakes fitted.

 

[Jan]

Your comments imply that the spacing from the distant to the stop signal is fixed and therefore increased braking is necessary at speeds above 140km/h. Is this something specific to the standards applying in Germany or some other country?

Yes, for whatever reasons Germany evolved to only use a limited number of fixed braking distances (400 m for up to 80 km/h (usually only on branch lines), 700 m up to 120 km/h and 1000 m originally up to 140 km/h, then extended to up to 160 km/h). Off-hand I can't really say how exactly that came about, though. After the war there might also have been some sort of co-evolution with our classic train protection system (Indusi/PZB), which after passing a distant signal at caution supervises a fixed braking curve, and therefore works best in conjunction with a fixed standard braking distance.

For a short while in the 70s, increasing braking distances to 1500 m and introducing double aspect signalling was being contemplated for allowing 200 km/h running on existing lines, but eventually it was decided to only eke out 160 km/h out of the existing signalling and require cab signalling for anything faster. This also means we've ended up with what is probably one of the most demanding braking curves under conventional signalling in Europe. It also means that whereas in the UK container trains running at 120 km/h/75 mph are common, in Germany 110 km/h – or rarely 115 km/h under favourable conditions – is the upper limit possible without cab signalling, and even then not on anything more than a minimal falling gradient.

I'm also surprised that eddy current braking is needed for operation on any new high speed lines, because I don't believe they are mandated in the TSI so such a requirement is going against interoperability.

It isn't an absolute requirement, but the Cologne - Frankfurt high speed line is somewhat of a special case because it was built with gradients up to 40 ‰ (1:25) in order to limit the amounts of bridges and tunnels required due to the hilly terrain there. Running at up to 300 km/h on that kind of gradients would place undue demands on the friction brakes (at least based on the brake technology available at that time and also DB's particular experiences and operating philosophy – I know that at similar speeds, French LGVs go up to 35 ‰, which isn't that much lower, plus the TGVs all still exclusively use power cars, which should be less suited to implementing high dynamic braking forces), hence the decision to equip the ICE 3 and its immediate successors (BR 406/407) with eddy current brakes. A few years ago, the absolute requirement for eddy current brakes on that route was dropped, but trains without them are currently still limited to somewhat lower speeds (around 230 - 270 km/h) depending on how powerful their regular dynamic brakes are.

 

[5920]

I am sure that the current demand of the electromagnets would be well within the capability of the unit's batteries. Tram track brakes are not dependent on the OLE supply.

On a 10 car Voltage drop can be into the 19s at the far end of the formation. On top of everything else hanging off the system added after the units were designed. I wouldn't be so sure before being confident of this.

 

[Greybeard33]

On a 10 car Voltage drop can be into the 19s at the far end of the formation. On top of everything else hanging off the system added after the units were designed. I wouldn't be so sure before being confident of this.

Surely the system would be designed so that the magnetic brake solenoids would be fed via a relay from the local batteries on each unit, not via wiring running the whole length of the formation? And volts drop would depend on the cable gauge used for the new wiring.

 

[rebmcr]

I'm also surprised that eddy current braking is needed for operation on any new high speed lines, because I don't believe they are mandated in the TSI so such a requirement is going against interoperability.

There is this (albeit uncited) line from the Wikipedia article for Eddy current brakes:

The TSI (Technical Specifications for Interoperability) of the EU for trans-European high-speed rail recommends that all newly built high-speed lines should make the eddy current brake possible.

 

[Taunton]

The magnetic track brakes on Manchester Metrolink and other light rail systems are used only for emergency braking, so track wear is really not an issue.

That's only a recent approach. The historic tram network in London, closed in 1952, used magnetic brakes for service stops ever since a government decree on tram braking in 1906. The howl of the brakes clamping down was a characteristic of every stop. They were not powered from the overhead wire supply, but by regeneration from the motors, so performed best at higher speeds. There's a working old London tram in the museum near Lowestoft, who will be I'm sure pleased to show it.

 

[507 001]

That's only a recent approach. The historic tram network in London, closed in 1952, used magnetic brakes for service stops ever since a government decree on tram braking in 1906. The howl of the brakes clamping down was a characteristic of every stop. They were not powered from the overhead wire supply, but by regeneration from the motors, so performed best at higher speeds. There's a working old London tram in the museum near Lowestoft, who will be I'm sure pleased to show it.

I'm surprised at that. The track brakes fitted to modern LRVs such as the M5000s I drive are very noisy.

As a side note, we're allowed to use them in emergencies (as part of a hazard brake application:- Rheo, Disks and Track Brake together, initiated by placing the TBC in the Hazard position) or on their own during low speed slides (initiated by pressing the button on the desk).