Axle Counters

[ole man]

Last week a LM 350 at Nuneaton ran over a sheet of metal which was either placed across the tracks or had fallen from the road above, this section of the WCML has axle counters, what i want to know is why the sheet of metal wasn't picked up by the signaller.
Could having axle counters instead of track circuits one day cause a problem, surely it should of registered with the signaller.

 

[Tomnick]

Obviously not the case here, but a sheet of metal placed in the middle of an Absolute Block section (in most cases) wouldn't be detected either. Although indicating certain obstructions is a handy side-effect of track circuits, remember that it's not by any means their primary purpose!

 

[swt_passenger]

I think the question you are asking is do normal track circuits reliably detect any random piece of metal falling on a rail, or across both rails?

Unless the metal sheet was completely corrosion, grease and paint free and making very good electrical contact with both rails you probably couldn't rely on track circuits to 'see' it - there have even been classes of train that couldn't be reliably detected in the past, IIRC.

 

[The Planner]

Maybe I don't understand axle counters properly, but surely they count axles in then minus the axles out at the next counter ?? why would a sheet of metal across the track flag anything up ??

 

[driver9000]

Metal object can and have been detected by track circuits when across both rails. Axle counter areas are not track circuited so wouldn't detect the metal sheet. Track circuit clips have no use in axle counter areas either from my understanding of their operation.

 

[Bald Rick]

Track circuit clips were invented as the last line of defence in the event of an incident where the signaller needed to be informed to block the line because of an incident but the driver was nowhere near an SPT.

Thankfully signaller to train radio has since been invented, and all axle counter areas must have this.

There are plenty of examples of metal objects falling on the line, and broken rails, which have not been picked up by a track circuit.

 

[TDK]

Track circuit clips were invented as the last line of defence in the event of an incident where the signaller needed to be informed to block the line because of an incident but the driver was nowhere near an SPT.

Thankfully signaller to train radio has since been invented, and all axle counter areas must have this.

There are plenty of examples of metal objects falling on the line, and broken rails, which have not been picked up by a track circuit.

a 158 wasn't picked up during leaf fall on the SW lines and there was a collision and that was track circuited, a broken rail will generally if broken completely break a track circuit.

 

[Bald Rick]

a 158 wasn't picked up during leaf fall on the SW lines and there was a collision and that was track circuited, a broken rail will generally if broken completely break a track circuit.

Broken rails only get picked up by a track circuit if a clean break (in my exprience about 1/3 of broken rails) and if in the signalling rail (by nature, half of broken rails).

 

[ole man]

Then surely there should be a system that could detect objects such a sheet of metal, what happens if a car is blocking the line?

Track circuits clips cannot be used in a axle counted area, though in your COSS course you are still required to block the line with TCC and apply Dets.

 

[Tomnick]

As I said, there's never been such a system in the middle of an absolute block section - so why should a new axle counter section (with shiny new radios as a bonus) be any different?

 

[Cherry_Picker]

Track circuits clips cannot be used in a axle counted area, though in your COSS course you are still required to block the line with TCC and apply Dets.

They say that to train crew too. The reasoning is pretty sound, it just removes hesitation where you could be wondering if you are in a track circuited or axle counted area (and if you have just hit something on the track after travelling for an hour or two at 100mph+ then it is understandable why doubt might creep in) so sticking 'em down and worrying about whether they actually did anything useful later is definitely the way to go.

 

[nr_signaller]

Axle counters are there to detect the prescence of a train, as are track circuits.

Track circuits do not reliably detect an obstruction on the line anyway. The obstruction would have to conduct electricity and also be touching both rails at the same time. What are the chances of that?

 

[jopsuk]

Then surely there should be a system that could detect objects such a sheet of metal, what happens if a car is blocking the line?

the dielectric of the rubber in car tyres varies but is generally pretty high- yes, lightning can overcome it, but it isn't enough to (for example) make a car safe to touch if a high voltage transmission cable is touching the body. The undersides of cars are coated- if they weren't, in this country, even galvanising the chassis wouldn't be sufficient to protect against road salt.

So, the underside and tyres of a car aren't that likely to make a good enough electrical contact to conduct the low voltage of a track circuit- remember, as said, that early disk braked units had trouble with track circuit actuation.

Anyone got access to a track-circuited line that they can set up a suitable possession to test my hypothesis that a car on the tracks is unlikely to actuate a track circuit? It's a scientifically testable hypothesis, and I would of course be happy to concede should it be shown that they can- though there are also shades of grey relating to likelihood, positioning and reliability of actuation.

 

[ole man]

Lets go a step further and say what would happen if a train de-rerailed across open tracks, would axle counters stop a disaster?. Would Track circuit areas pick up the train?

 

[Tomnick]

Lets go a step further and say what would happen if a train de-rerailed across open tracks, woulod axle counters stop a disaster?. Would Track circuit areas pick up the train?

You certainly couldn't rely upon the derailed train occupying track circuits on the adjacent (obstructed) line - a decent radio system, which is required in the new axle counter areas (whether it works as such is a different matter!) would probably be more reliable. Compare that to an absolute block section, where you have neither...