Railway signalling discussion

Aberdeen Railway (Drumlithie)

British signalling of the past (UK, excepting Northern Ireland)

Re: Aberdeen Railway (Drumlithie)

Unread postby John Webb » Sun Dec 31, 2017 5:00 pm

Mike Hodgson wrote:..... but would it really be practicable to renew one side of a timber bridge whilst keeping the opposite line safely open to traffic? ...... were they two separate but parallel viaducts?

I assume this is the bridge concerned (click on picture to go to the larger original):
Bervie Water From Old Mondynes Bridge

© Copyright Dominic Dawn Harry and Jacob Paterson and licensed for reuse under this Creative Commons Licence.

It isn't clear if the stone piers supported the previous timber viaduct, but it they did it might well have been possible to remove one side of the viaduct and replace in stages; there are only two spans.
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Re: Aberdeen Railway (Drumlithie)

Unread postby Colin Wells » Sun Dec 31, 2017 9:46 pm

Risking the wrath of the Administrators for a moment and continuing the theme of bridge construction, a Google search reveals a plan of the replacement bridge that appeared in a Railtrack calendar in 1998 at As can be seen, also from the above photo, it's a viaduct but not of the type that usually springs to mind. The National Library of Scotland Ordnance Survey map pages eg,-2.3457 show much the same configuration of bridge between 1860 and 1901 - two double line spans with a pier in the river. I would speculatively agree that one side of the bridge deck could have been retained whilst the other was in process of renewal.
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Re: Aberdeen Railway (Drumlithie)

Unread postby Andrew Waugh » Sun Dec 31, 2017 10:16 pm

Mike Hodgson wrote:This explanation certainly sounds plausible from a signalling point of view but would it really be practicable to renew one side of a timber bridge whilst keeping the opposite line safely open to traffic? There may not have been much consideration over Health & Safety of the workforce back then, but surely there would be some concern over the risk that the works would undermine or otherwise foul the line still in use? I don't know the location - were they two separate but parallel viaducts?

Assuming that the bridge design was suitable, absolutely it was possible. Victoria doesn't have many major viaducts, but quite a number have been renewed in exactly this way. One, indeed, was renewed in this way at the same time as this viaduct.

You couldn't close the line while a major renewal was being undertaken. So there were only two alternatives; build a completely new bridge parallel to the old, or renew the girders piecemeal under traffic. I would imagine quite detailed engineering calculations were made of the options.

Dragging this back to signalling, the signalling system would have been used to protect the worksite. The Victorian contemporary example opened a signal box at the bridge, which was a non crossing electric staff station. The steam crane used to renew the girders could only be operated if no trains were approaching the bridge. Further, sidings were provided on each side of the bridge to refuge the crane. These were Annett locked. Removal of the Annett key locked all signals at stop, and the crane had to carry the key while it was on the bridge.
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