Background

About three years ago I decided to take up railway modelling again after a break of about 35 years – the usual reasons: children and career followed by early retirement! The standards of all aspects of the hobby have improved so much during my time away that I was very apprehensive about my ability still to produce models of exhibition quality. Before trying to build my own layout, I decided to build a diorama to test my model making skills, before committing myself to a larger, more expensive project.

I decided to attempt to build a model of one of Brunel’s timber viaducts and started my research. The standard work on the subject is John Binding’s aptly named “Brunel’s Cornish Viaducts” and this allowed me to make a choice of which viaduct to model. My choice was based on the length and height of the various alternatives, but having decided upon Treviddo viaduct I soon discovered that it had been well photographed and, in particular, one photo had been used on a commemorative postage stamp to mark Brunel’s bicentenary in 2006, (or to be accurate on a Post Office mini-sheet.) My project therefore morphed into a diorama which would attempt to recreate the picture on this stamp.

A visit to Steam at Swindon produced the original of the photo used on the stamp and for about 18 months on and off I enjoyed myself greatly doing my research, in Cornwall, Kew, Swindon and York. It was a real thrill to be handling documents which had been used by Brunel and his staff 170 years before. I had to suspend starting work on the diorama for a bit to leave time for other projects and family activities, but was recently motivated to re-start by receiving an email out of the blue from a modeller in Australia asking me for my help in researching Brunellian viaducts!

Drawing the Plans and First Steps

By careful reading of Binding’s descriptions, together with information in Brian Lewis’ book “Brunel’s timber Bridges and Viaducts” and my own researches, I was able to draw up some quite reasonable plans to a scale of 1mm to the inch. This allowed me to get clear in my own mind the geometry of the various timber joints in the structure.

I have always regarded the diorama as an experimental project which would allow me to try out different ideas and methods to see which worked best. So the first job was a suitable baseboard, and this one presented particular challenges.

The viaduct was 486ft long – 1944mm in OO – and I wasn’t prepared to have a baseboard join half way along! This meant a rather long baseboard which would have to be very stiff and also resistant to torsional forces. I could quite imagine the effect on the matchstick-like bridge superstructure of twisting the baseboard! I eventually decided to make the baseboard 2140mm long – 7ft and ¼ inch – it seemed appropriate.

Some measurements from the original drawings in Truro records office and the ordnance survey maps allowed me to draw out some longitudinal and transverse sections of the valley in which the viaduct was built. I’ve now cut them out in 9mm ply and checked that they fit together. They need a little bit of fettling, but eventually these will be the formers on which the ground surface is modelled.

What I need to do now is find out how much of the formers I can safely cut away to save weight.