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Why does the third rail change sides?

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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.

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