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Why does the third rail change sides?

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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.

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