Chris M
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Post by Chris M on Dec 6, 2009 2:25:54 GMT
Several times on this forum, including recently on a Piccadilly Line thread, I've seen confusion and disagreement about when a bridge is an overbridge and when it is an underbridge. The answer is that a bridge is usually both, it just depends on what your perspective of it is. Hopefully this will explain things the way I understand them to be - ![](http://www.sucs.org/~cmckenna/diagrams/HillingdonBridgesm.jpg) (click for a larger version) In this photograph looking east from Hillingdon station, there are two bridges, one in the foreground carrying the railway over the A40 Western Avenue, and one in the background taking the B466 Long Lane over the railway. This can be simplified into this beautifully ![;)](//storage.proboards.com/forum/images/smiley/wink.png) coloured diagram ![](http://www.sucs.org/~cmckenna/diagrams/HillingdonBridgediagram.png) The railway is red, roads are green, the bridges are orange and blue, the sky is white and everything else is grey. For a railway engineer, there is one overbridge (blue) and one underbridge (orange). For a highway engineer, there is one overbridge (orange) and one underbridge (blue). For the railway engineer, the orange bridge is an underbridge, because it takes something (a road in this case) under the railway. For the highway engineer, the orange bridge is an overbridge because it takes something (a railway in this case) over the road. The same logic applies to the blue bridge - a railway engineer sees it as an overbridge carrying something over the railway, the highway engineer sees it an underbridge, carrying the road over something. Does that all make sense?
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Deleted
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Post by Deleted on Dec 6, 2009 4:18:32 GMT
I think this is a very good explanation - thank you!
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Post by citysig on Dec 6, 2009 8:49:03 GMT
Normally, whether it is a rail or highway engineer reporting anything to do with bridges, they are handed to us with the appropriate extra word that details exactly what the bridge is.
So in the case of the orange bridge, it would be reported as a "Rail Overbridge" or a "Road Underbridge." This simple method quickly identifies who is on top and who is underneath, regardless of the reporting person's perspective / interest.
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Ben
fotopic... whats that?
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Post by Ben on Dec 6, 2009 16:07:32 GMT
Very consise and useful explanation, a nice bit of pop art, and an extremely familiar view! ![:)](//storage.proboards.com/forum/images/smiley/smiley.png)
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Post by ruislip on Dec 6, 2009 21:59:20 GMT
Very consise and useful explanation, a nice bit of pop art, and an extremely familiar view! ![:)](//storage.proboards.com/forum/images/smiley/smiley.png) Yes--weren't the old Hillingdon platforms located where the line crosses over the A40?
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Phil
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RIP 23-Oct-2018
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Post by Phil on Dec 8, 2009 22:53:44 GMT
Yes, the railway (NR) terminology defines the road. So an "underbridge" refers to the road going under the railway (i.e. a road underbridge) and vice versa. ChrisM put it nicely ![;)](//storage.proboards.com/forum/images/smiley/wink.png) .
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