Track Circuits (rural, mainline, metro)

[Foghut]

Originally Posted by RailProject

From what I've read and looked up it seems that most mainline railway lines are powered by 25kV AC 50Hz. Am I correct in this?

Hmm, I'm not sure I'd say most. Someone on here may be able to tell you just what proportion of route miles in the country are OHLE/3rd Rail/Not Electrified.

3rd rail 750v DC accounts for nearly all of the South Eastern Region, Southern Region, and South Western region.

As you pointed out, we would need to know the 'noise' generated by the traction systems/electric motors and also the noise generated by the electrification system.

Sure, and it's not just noise. You would be looking at some hefty traction currents in the running rails. Several hundred amps per commuter train, considerably more for locos and intercity stock.

Once these are known we should theoretically be able to filter them out. I've been searching forthis info but I honestly have no idea where to start! There is just so much information around. Any pointers....

Why not speak to the guys currently doing the job; Network Rail's Signalling & Telecom engineers ?

I have also been trying to find the characteristics (generally) of a railway line in terms of impedance and admittance (resistance, inductance, conductance and capacitance). With these figures we should be able to work out the 'best' frequency to send down the line.

I have absolutely no idea what 1 km of steel rail looks like electrically, but from the postings earlier on this thread you can see that the characteristics do change significantly, particularly due to dampness in the ballast.

HTH,
Foggy

[hairyhandedfool]

I'm not an engineer but I know model railways sometimes use infra red beams to detect trains passing signals but how that could work in real life is anyones guess, badly I suspect.

[Shed Cat]

Originally Posted by hairyhandedfool

infra red beams to detect trains passing signals but how that could work in real life is anyones guess, badly I suspect.

The question is how you would be 100% sure that it was a train that was tripping the beams and not a bird, wandering badger or a couple of kids waving branches in front of the detectors.

But it would have to be sensitive enough that every time a real train passed it would trip the beam.

[Foghut]

Well there already is a system for getting instantaneous indications of a train's presence - treadles. You don't get false readings with these as they require the flange of a train to operate them. (AFAIAA these tend only to be used where track circuits can't be such as in the middle of complex junctions, or to trigger events like occupational crossing warnings).

[hairyhandedfool]

Originally Posted by Shed Cat

The question is how you would be 100% sure that it was a train that was tripping the beams and not a bird, wandering badger or a couple of kids waving branches in front of the detectors.

thats true, you don't get many of them on model railways, so it didn't cross my mind!

[Derbyroy]

Good thread this one,
I have like a few others have tried the national database for rail signalling and track circuitry, it appears that because the system was inherited by railtrack, from the former British Railways, no actual plan or scematic, is held on record anywhere except at the former regional depots of the big four companies, lol. seems funny that an infrastructure essential like this could be left so low on the list of essential paperwork to be centralised ?
I would like to know the answer to the questions above though , it seems we may all be safer if this matter was brought to the attention of the people responsible for our safety whilst on the rails surely ?
however
things are bypassed daily so i won,t hold my breath
best regards Derby

[RailProject]

Quick question:
Impedance bonds. Are they basically a piece of equipment which provide a path for traction currents to pass through, but still keep track circuits electrically isolated?

I just want to make sure I understand them correctly.

[Foghut]

Originally Posted by RailProject

Quick question:
Impedance bonds. Are they basically a piece of equipment which provide a path for traction currents to pass through, but still keep track circuits electrically isolated?

Basically yes. They're known coloquially round here (SE England) as spiders, and we're told that the postion of the spider does not always mark the end of the track circuit !!

As usual Wikipedia has something to say here... http://en.wikipedia.org/wiki/Track_circuit

But remember that Wiki tends to tell things like the Americans do it

HTH,
Foggy

[RailProject]

So, the project has now been broken into several areas.

I decided to take the section we have called Filtering.

Basically the idea behind this sub-system is that it will filter the received signal of any noise and be left wit the reflected pulse which can then be used to determine train position.

My first obective is to identify all the possible areas were noise coild be created which could be imparted into the rails (therefore into our square wave (with highliy skewed duty cycle))

Below is a short list of what I think could be the main causes for noise within rails:
- Traction noise due to rectification
- Electrical equipment onboard train such as air conditioning
- power converters or other on-board equipment
- Mobile Phones?
- Overhead power lines in close proximity to rails

The traction noise is probably going to be my biggest source of noise right? As this is only a project and not an actual design I am thinking of limiting my outlook to only newer trains with IGBT controlled induction motors.

What I need to find out is what this noise looks like (electrically) so that I can start thinking of different ways to filter these out from my wanted signal.

If anyone has any information regarding these sources of noise and possible ways of filtering the noise then I would appreciate any input.

Thanks!

[Foghut]

Perhaps you should approach Bombardier to find out what sort of electrical products are passed into the running rails by a modern train. IIRC they make the 375/376/377 series EMUs that are taking over the South East at the moment.

Link to them at - http://www.railway-technology.com/co...ier_transport/


Also since what you are doing is mildly similar to radar with aircraft, it might pay you to have a look at what sort of software/DSP is already developed in that field for distinguishing between the 'return' and the 'clutter'.