Dom K
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Post by Dom K on Sept 10, 2015 17:50:35 GMT
Talk here about this subject
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Post by kesmet on Sept 17, 2015 18:50:49 GMT
Following on from the suggestion about inter-running being made easier if the four-rail system was abandoned, I thought I post some of my thoughts on the subject.
Four rail has a number of advantages over three rail. You can isolate running mechanisms from powering mechanisms, which should reduce complexity. I'm wondering if this makes it 'safer' - by enabling the train body to be an "earth"? Also, there's the ability to have simpler signalling mechanisms - no need for audio-frequency AC systems if the power return isn't in the running rails. And with each rail having a lower potential difference from ground, it should be overall safer.
Inter-running is also harder than just similar electricity delivery. There's signalling, as norbitonflyer mentioned, and loading gauge works both ways. I don't think that all Tube stock is safe to run on mainline metals even without signalling or power considerations; there's lots of 'stuff' that might get in the way - either on the train or on the track.
Does anyone know if the bogey positions on Tube stock are safe for UK tunnels? Would the ends of the cars always be clear of the walls?
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Chris M
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Post by Chris M on Sept 18, 2015 12:05:37 GMT
Does anyone know if the bogey positions on Tube stock are safe for UK tunnels? Would the ends of the cars always be clear of the walls? That will depend on the stock and the tunnel. If the tunnel is cleared for container traffic I would expect fewer issues than in the tunnel(s) near Hastings that required narrower than usual stock. |
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Post by bananaman on Sept 18, 2015 12:52:44 GMT
You can isolate running mechanisms from powering mechanisms, which should reduce complexity. I'm wondering if this makes it 'safer' - by enabling the train body to be an "earth"? The train body is at earth potential in both systems. In a three-rail system, one leg of the traction supply has to be at earth potential. With a four rail system neither supply leg is tied to earth. Also, there's the ability to have simpler signalling mechanisms - no need for audio-frequency AC systems if the power return isn't in the running rails. Using audio frequency track circuits, amongst other things, allows the circuits to be "tuned" to the impedance of the rails. This does away with the conventional insulated block joints that are a common cause of track failures. |
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Deleted
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Post by Deleted on Sept 18, 2015 12:57:20 GMT
This is only a recent introduction of getting rid of the blockjoints. They still have JTC track circuits running in jointed mode.
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Post by bananaman on Sept 18, 2015 13:06:26 GMT
That depends on what line you're on  |
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Post by norbitonflyer on Sept 18, 2015 13:29:42 GMT
I would expect fewer issues than in the tunnel(s) near Hastings that required narrower than usual stock. The lines through those tunnels were singled when the line was electrified, so that standard-sized electric stock could be used. The reason for the narrow profile in the first place was that the contractors building the tunnels used a single course of bricks for the lining instead of the double course specified. The only way the replacement contractors could rectify the problem once it had been identified was to lay an extra course of bricks within the diameter of the existing tunnel, thereby making it narrower by the thickness of two bricks. |
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