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Author Topic: Why does Cholsey bridge have such a tall hump?  (Read 415 times)
Marlburian
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« on: November 28, 2022, 06:26:14 pm »

Today I walked alongside much of the Wallingford branch and wonder why Cholsey Bridge was so tall. The original terrain is flat and steepish ramps had to be built on either side, making it an uphill plod for horse-drawn vehicles. (And when motor traffic came along there must have been some near misses between vehicles coming to the brow at the same time from different directions- nowadays there are traffic lights.)

Ignorant layman that I be, I would have thought that the bridge could have been some eight feet lower.

Any better-informed thoughts?
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Oxonhutch
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« Reply #1 on: November 28, 2022, 09:14:55 pm »

why Cholsey Bridge was so tall?

Was it perhaps originally built for the broad gauge? Yes I know the branch was one of the first SG ones, but ...

Anyone have a similar photo of the old Hithercroft Bridge - now gone - beyond the end of the current line?
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stuving
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« Reply #2 on: November 28, 2022, 10:07:15 pm »

why Cholsey Bridge was so tall?

Was it perhaps originally built for the broad gauge? Yes I know the branch was one of the first SG ones, but ...

Or, alternatively, based on the same starting point, is it narrow compared to other GWR (Great Western Railway) bridges you are familiar with, so looks taller than it is?
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grahame
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« Reply #3 on: November 28, 2022, 11:01:09 pm »

Or could it be that the cutting is deep, so the deck of the bridge higher than usual?
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stuving
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« Reply #4 on: November 28, 2022, 11:47:19 pm »

Or could it be that the cutting is deep, so the deck of the bridge higher than usual?

No, because the formation is close to natural ground level.

The 1876/1897 OS (Ordnance Survey) map on NLS shows spot heights on the road about 300' either side of the bridge, at 162' and 155'. It also shows a benchmark at the bridge of 176.4', which I would expect to be low down on the parapet (who fancies going poking about in the undergrowth for that?).

It looks as if the ground slopes pretty evenly, so let's say the natural level at the bridge is 158-159', which places the benchmark 18' (plus or minus a foot or two) above that. The typical structures gauge is a minimum of 4m above railhead, so assume the bridge was built to be above that level, say at 13-14'. To that must be added the height of the railhead above ground level (1'), an allowance for the narrow arch (1'), and the depth of the masonry up to the benchmark (say 3'). That adds up to 18-19' - so I would say the bridge is exactly the expected height.

Big things, trains.
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Marlburian
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« Reply #5 on: November 29, 2022, 08:55:58 am »

You're right, Stuving, I guess I was misled by the bridge being quite narrow. I can't recall seeing many bridges built over a single-track railway on level ground; in most cases the track has gone through cuttings - or there's been a level crossing.

I came across the Forgotten Relics website, which has more content referring to tunnels rather than bridges, and this shows portals with high headroom.

My walk took me under Moulsford Railway Bridge, after I'd passed the former Fairmile Mental Hospital, the site of which has been recently redeveloped, with the original accommodation being turned in apartments and new houses built around it. I was quite impressed - though the former chapel has yet to be converted.
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bobm
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« Reply #6 on: November 29, 2022, 03:24:37 pm »

I thought it was to allow for future electrification.   Grin
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Bmblbzzz
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« Reply #7 on: November 29, 2022, 03:28:18 pm »

I thought it was to allow for future electrification.   Grin
Possible. But more likely it was for the overhead pneumatic drive tube.  Shocked
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