Why does the third rail change sides?

[stuartwaters]

Excellent stuff, it's good to read stuff from people in the know about these things. I used to travel by train to London daily in the mid-80s and often wondered in my boredom (in the days before smart phones) about how it all worked.

[grandarog]

Interesting thread. I don't travel by rail much these days . When I was at School I travelled daily and also a lot when I was in the RAF.  I remember counting  as we passed the little marker posts at the trackside to work out the speed we were doing. Timed I think  passing 4 .Cant remember the Formula to get the speed ,no doubt one of you will know it. No third rail much in those days so had no idea it changed sides .

 

[MartinR]

I know we are drifting OT here, but just out of interest why the change to axle counters?  Is there any equivalent to the track circuit clips?

[John Walker]

This is fascinating stuff.  I never knew it was so complex and assumed it was just a matter of supplying a current to the third rail.  How wrong was I!

[Stewie]

Typically DC track were used in AC overhead areas but need insulated joints in the rail which upsets the PWAY engineer when they use continuous welded rail. Modern track circuits are frequency and or digitally coded so as not to interfere with each other or be affected by traction return from the trains. Boundaries between different tracks are made by shorting out the frequency of the adjacent track in a tuned zone of between 5 to 20 metres. Track circuits are being replaced now by axle counter technology which counts flanges into and out of a section of track and compares the result before clearing.

[MartinR]

Thanks Stewie.  I've only come across filters in electronics and in the supplies to my computer rooms.  Low pass inductors get pretty big when they are blocking AF in circuits, and for the computer room supplies (100-200A/ph) they are boxes maybe 1' square by 3' high for each phase.  Which is why I was surprised by the simple comment.  Thinking about it though, for track circuits you have the advantage that you are not needing a low-pass filter but a notch type tuned to the block's frequency.

Is there a reason that the Hastings line would have the live rail on the outside?  I can remember when it was electrified through Tunbridge Wells and the railwaymen that frequented the The Bedford talking about keeping the 6' clear for safety.  That  line is a little funny though; it used to have specially slimmed down stock for the narrow tunnels, and after electrification I understand some of the tunnels had to have a sort of single line working.  I also notice that London Underground, when overground, seems to put the +420VDC rail outside the 6'.

You mention specifically that "a railway using DC traction, track circuits are AC".  Are there any DC track circuits these days?  With the advent of continuous welded rail I thought that all track circuits were AC but the blocks tuned to different frequencies.  Would there be an advantage is using DC circuits when on the AC overhead?

[Stewie]

In simplest terms. For a railway using DC traction, track circuits are AC. At a track paralleling points, the AC track circuit signal is passed through two contra wound coils tuned with a capacitor to cancel out the AC signal. The DC traction currents are piped straight through via the TP hut. This all occurs in a large black box mounted between the running rails in the ‘4 foot’. The piece of equipment is called an ‘impedance bond’. The net result is that the tuned circuit gives a very high AC impedance (and prevents the track circuit being shorted out) and a low DC resistance allowing it to flow easily.
In my experience, conductor rails are generally in the 6 foot unless there is a reason for it not to be. When you are exiting the track before the arrival of approaching train, stepping over the running rail and the conductor rail is not nice.

[John Walker]

Thanks again for the explanations.

The attached photo is an example of the third rail changing sides.  Here the third rail has changed to the 'outside' and there is a building in the centre of the section.  This is on a long straight section of track.  There are other examples along the track but I'm not certain there's a building beside each one.

Also, note that the up and down lines do not have these changes in the same places along the track.

[MartinR]

Stewie, thanks for the 8'/6' correction, I must have been having a senior moment!  JW referred to "a small building around the halfway mark", not "large brick built buildings", perhaps we need clarification from him as to his definition of "small".  Could you clarify "electrically connect the running rails in parallel to allow for the traction return to have the lowest possible resistance path back to the substation"?  AIUI connecting the rails together would mess up the track circuits.  Electronically it is possible to block AC whilst allowing DC through, but I personally would be surprised if this could be easily done at the currents involved - perhaps you can clarify?  After all track circuit clips have to be effective.  One last thing, the observation is that on long straight stretches the conductor is on the outside, except for short stretches, if wear was the issue would not the lengths be equal?  Can you help?

[Stewie]

The third rail changes sides to even out wear on the train pick up shoes. It also allows for a train on a curved section of track to exert pressure on the third rail when leaning particularly if the track is canted. In stations, the third rail is placed in the ‘six foot’ between running lines for safety reasons. The third rail is fed from substations situated roughly every three miles along the line and these are probably the large brick built buildings that JW refers to. Between substations, smaller’ track paralleling’ huts electrically connect the running rails in parallel to allow for the traction return to have the lowest possible resistance path back to the substation. Each separate section of conductor rail may be isolated by locally operated ‘hook’ switches or longer sections may be switched from the controlling electrical control centre. Some sections are gapped to allow for natural interruptions such as level crossings, but the maximum gap must take into account the shortest distance between pick up shoes on a vehicle (usually a light engine) so tha it does not become isolated or ‘gapped’ on a dead section. The lifting span on the Kingsferry bridge at Swale has no third rail and so trains are required to be either multiple units or two locomotives coupled together.

[MartinR]

Those huts may simply be platelayers' huts where track maintenance equipment is stored and platelayers can seek shelter/have a brew.  I think most switchgear is trackside rather than in a building, but I may be wrong in this.  You're quite right that it is a DC system: 750 VDC.

[John Walker]

Thanks for the explanations.  When traveling on a train, you see a section of third rail appear on the offside at regular intervals once you get out into the countryside.  There always seems to be a small building around the halfway mark.  Could it also be where the current is boosted as I believe it's a DC current?

[MartinR]

The third rail is usually outside the tracks on normal double line, but inside in stations etc.  This is a safety issue, the so-called 8' 6' is the gap between two running tracks.  If crossing the tracks it is better to have the hazards of the third rails well separated and in an emergency it is safe to lie there.  Within stations you want the rail as far from the muppets passengers as possible.  turning to the observed brief changes of side, I believe this is so that sections of track can be de-energised.  You also do not want a the pickups on the front of a train to energise the rail from the pickups at the back.  Therefore to work safely on a section, de-energise it and the two short linking sections either end.  Of course no train should be attempting to enter a section where there is an engineer's possession, but accidents do happen.

[mmitch]

Usually its points. But I suspect it is also to even out wear on the pickup shoes.
Interesting when crossing a set of sidings, but that was many years ago!
mmitch.

[John Walker]

I've noticed that every so often, the third rail switches to the outside for a short section. 

Can any of our railway-buff members tell me why?