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 -
(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
coloured diagram
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?
Hopefully this will explain things the way I understand them to be -
(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
coloured diagramThe 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?
