Modelling a Brunel-Style Tunnel Mouth

Modelling tunnel mouths has always been a limited activity for the modeller who has generally had to resort to using commercially-made products such as the Peco/Merit or various card offerings.

This article presents a method of modelling a characteristic Brunel-style tunnel mouth. The techniques can be equally used on tunnel mouths and other constructions such as buildings, regardless of the prototype region.

Like many modellers, when I first started my layout some years ago, I opted for the Peco double track tunnel mouth:

However, the more I looked at this product on the layout, the more I realised that it really didn't look right and with this, I undertook some research to find a prototype for the Peco product.
I came to the conclusion that the Peco product is totally wrong: I was not able to find any photos on any of the UK regions of a tunnel mouth contructed like this. The nearest I came to it was a single bridge at Trethall Halt on the Helston branch in 'West Country Branch Lines' by Peter W Gray, page 76:

Photo: Courtesy Peter W Gray

The problem with the Peco product is that the retaining walls are wrong.
When a real tunnel was constructed (assuming a dug rather than 'cut and cover'), digging continued to form the cutting until it was deemed that the ground above was sufficiently hight and stable that tunnelling could commence. Other engineering reasons were also involved.
Modellers are often unaware that railway cuttings and embankments never slope at more than 1 in 1 and a half gradient unless the ground is retained or it is rock. This is because a real embankment/soil will not naturally stand up at greater than this gradient. A typical cutting will be V shaped right up to the tunnel mouth.
For the Peco tunnel mouth to be plausable, a 'quarry' would need to be cut out around the tunnel mouth in order to maintain ground stability, given the shape of the retaining walls. When I first built my cutting, I did exactly this as can be seen in the photo above.

There must be a better way!

For some years now, I have been using a method of plywood frame construction, covered with plaster and then scribed with a stonework effect with great success. I decided to employ this method for a tunnel mouth.

The first thing which needed to be done was to get the shape and size of the tunnel itself correct and to do this, I used the following picture:

Photo: Dainton Tunnel, Eastern Portal [Courtesy of SouthWest Trains cab views web gallery]

This was to be printed out and used as a template. The first thing which became obvious on printing the template was that as modellers, our curves are much sharper than prototype (in this instance, mine are 5 foot radius) and we place our tracks at a greater separation than prototypical. I therefore used computer software to enlarge the template, maintaining the aspect ratio, until when printed, the width of the tunnel mouth spanned my tracks with sufficient clearances. So although the model is not an absolute to scale model, it is fully in proportion.

This is the template:

The template was then used to mark out the tunnel mouth on 3mm ply. The Eastern portal of Dainton was chosen because in comparison, the Western Portal is plain in appearance. The Eastern portal is also fairly unique in that it is flat-fronted which makes it easy to model, unlike the western portal which is curved.

Following is a series of pictures showing the construction of the frame, followed by plastering and the finished product. Painting is a mixture of watery washes of water colour which gives a very reallistic stone effect. Some Carrs powders have also been used to give 'tone' and to mark the soot on the top of the arch:

The hillside on the layout was opened up and a suitable siting base was built. The tunnel mouth was put in place and a series of tests were undertaken with various items of rolling stock used to test clearances.
Initially, the mouth was sited too low on the ground such that rolling stock roofs collided with the tunnel 'tube'. Quick checks against pictures confirmed that the mouth needed siting higher, which was done.
There's nothing worse in my view than looking in a model tunnel mouth and seeing the inside of a hill! A tunnel 'tube' was therefore added, although it isn't shown in the pictures below because some minor trackwork was in progress at the time.
A BR MKIII, a GWR Centenary and a GWR Autocoach were used to check clearances:

MKIII coaches are actually as good as MKI's in terms of overhang and end-swing, therefore, they were of little use in the tests. The two GWR coaches are very wide and were good tests.

Once position and clearances were confirmed, the tunnel mouth was fixed in place and the hillside rebuilt using fibre insulation board supporting chicken wire (which was pinned to the fibre board) onto which newspaper dipped in builder's finishing plaster was laid over with many, many layers, giving a very firm surface:

The ground around the tunnel was painted brown and then scenic work commenced:

The finished product appears below. Note the drainage covers either side and in between the tracks. These are very rarely modelled by modellers. In this instance, a drainage pipe would run down the centre of the tunnel with the centre cover acting as a 'T' piece with pipes under the track at right angles to the outer drainage pits.

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Graham Plowman

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