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For me one of the most fascinating aspects of model railways are the civil engineering structures such as bridges and viaducts. On an indoor layout these are typically constructed of card or other lightweight materials over a wooden framework or former and are often finished to a high level of detail. However, for an outdoor layout we need something much more resilient from which to build our structure, something that can withstand the rigours of extreme weather, and in my opinion one of the most durable and visually realistic products we can use is stone, or more accurately, a standard lightweight building block.

An outdoor layout isn't normally finished to the same high level of detail that we have come to expect from a similar layout housed indoors and so construction techniques are often quite different. I would imagine there are very few indoor models constructed using a mixture of sand and cement, certainly none that would be easily transportable, but such materials are the minimum requirement for extended outdoor use.

Although the technique mentioned here would be equally suited to the construction of a variety of bridges, for the purpose of this article we will be concentrating on the construction of a stone viaduct. However, before we take a look at just what's involved it may be of interest to look back at my earlier attempt at making an OO gauge model railway viaduct from plywood so that you can understand why I would now highly recommend the use of stone.

Sounding much like the title to a classic song, back in the summer of 2009 I finally realised my ambition of one day being able to construct a large model railway layout and from the outset I had decided that the layout, to be located by necessity outdoors, had to incorporate a viaduct. I had never built a model railway before but decided that the viaduct itself would be constructed from exterior grade plywood. The photograph below shows that particular viaduct under construction despite the fact that it appears to be more in a state of demolition.


As you can see from the above photo, the main structure of the viaduct was constructed from 9mm exterior grade plywood with the arches cut out using an electric jigsaw. Plywood spacers joined the two sides together creating the inner arch walls and short sections of plastic guttering formed the curved roof of the individual arches. The viaduct was constructed in two separate sections, the longer section consisting of 11 arches and the smaller section with just 5. A short over-bridge, designed to resemble that spanning a waterway, connected the two sections together into one long structure with an overall length of 10 feet 6 inches. The full extent of the viaduct can be seen in the following photograph, again taken whilst still under construction.


In an effort to make the viaduct look more realistic and in order to help protect it from the weather, I decided to cover the facing side of the plywood with a thinly applied coat of exterior grade Polyfilla. This was done with limited success but while I was happy with my first attempt it was clear the the plywood structure would be susceptible to the rain and so whenever the layout wasn't in use the viaduct itself was covered by a large tarpaulin to prevent the track bed becoming waterlogged. The final photograph shows the Polyfilla covering I applied to the plywood sides which did give the effect of the stone structure I was looking for but also highlights my rather crude attempt at adding a girder bridge across the central span.


Rather abruptly, an unforeseen house move meant that the entire layout had to be dismantled before it ever reached completion and the plywood viaduct itself was consigned unceremoniously to the local recycling depot. The good news was that the move of home enabled me to begin the construction of a new outdoor OO gauge layout, one that would again include a viaduct, but with previous experiences to hand it was decided this time to build in a more robust material.

My first thoughts for a new viaduct were that it might be possible to cast one completely in concrete. I've seen descriptions of structures made that way before using wooden shuttering to constrain the concrete mixture and polystyrene block formers for the arches which are later removed once the concrete has set sufficiently but it seemed like a lot of work with no guarantee of a perfect finish. Could there be an easier way...?

I had almost resigned myself to building a viaduct out of cast concrete when a member of the OO Garden Railway forum suggested the use of Thermalite blocks for the track base I was building at the time. The track base required raising just a few inches off the ground and concrete foundations to such depth would have looked unsightly and so following that member's advice I purchased several lightweight Celcon aerated blocks which were described as being suitable for outdoor use. I hadn't realised just how lightweight those blocks actually were, or how easily they could be cut using just an old saw. It soon became apparent that they could also easily be shaped using a coarse file or rasp. It was then that I had the idea of using this type of building block to construct my new viaduct. It would hopefully be structurally very sound and in addition it should be fully weatherproof.

A number of lightweight standard Celcon aerated blocks were purchased from my local DIY superstore and at less than £1.40 each they ultimately proved to be a very economical way to build a substantial viaduct structure. I decided to use the blocks in their upright position standing them on their shorter edge. The central section of each block would form a viaduct pillar and a narrow section from the lower two-thirds part of each edge would be cut out to create an half-arched shape leaving the cut block resembling the letter 'T'. The photograph below indicates where those cuts were to be made:

Low Shott Viaduct 01.jpg

As already mentioned, the blocks can be cut with very little effort using an old handsaw but that's just my personal tool of choice and I would imagine that most other types of saw would be equally suitable. A straight cut is made with the saw from the base of the block up to the line previously marked some two-thirds of the way up the block before an angled cut is made from the edge of the block to the recently cut line enabling the edge section of the block to be completely removed. You should eventually end up with something like this:

Low Shott Viaduct 02.jpg

The 2 edge sections can be put to one side - you might find a use for them later. Once you've completed a few more blocks and placed them side by side you'll find you have something that's just beginning to resemble a viaduct type structure.

Low Shott Viaduct 03.jpg

Okay, it's not quite a viaduct just yet but I'm sure you can see we're getting somewhere. How you progress from here is a matter of personal preference but I'll describe the way I chose and leave you to decide what's best for yourself.

In a similar way to building a brick or block wall stand the cut and shaped block on end and cement the next block up to it. If your ground is soft then you may need to add a shallow concrete footing to prevent later ground movement disturbing your viaduct but because my ground was very rocky I simply added a good layer of mortar around the base of the pillars to secure them in place. You may find that one of the modern contact adhesives is sufficient to hold the blocks securely together at the top but in this instance I chose to mortar them in position.


As you can see above, the first 5 blocks have been cemented in place with a narrow mortar gap between them whilst one full block on edge simply prevents any sideways movement until they have thoroughly dried. The base of the pillars is packed around with a mortar mix to hold them securely.

Once the blocks have dried you are left with a very solid structure and you can begin using a coarse file or rasp to round out each individual shaped archway.


Continue adding further blocks and rounding out the arched shapes for the full length of your viaduct. The more blocks you cement together the more impressive your viaduct begins to look, especially from the more unusual viewing angles.

Low Shott Viaduct 03b.jpg


To improve the appearance of the flat top of the viaduct I cut small individual sections of aerated block and glued them in place to create a narrow overhanging ledge and rising sidewalls. These can be seen in the above photograph before being easily shaped using nothing more than a sheet of sandpaper.

Low Shott Viaduct 22.jpg

To prevent the top of the viaduct becoming waterlogged, drainage holes were drilled at an angle downwards through the top of the viaduct so that they emerged on the sidewalls above each viaduct pillar. Once completed the whole viaduct was then treated with a water repellent solution suitable for application to bricks and blocks in order to limit water ingress. You can see where the drainage channels, drilled at an angle into the top surface, emerge through the sidewalls of the viaduct at the top of each pillar in the photograph below.

Low Shott Viaduct 21b.jpg

If all goes to plan you should end up with a very solid viaduct that will withstand all that winter has to throw at it without the need for the protection of a tarpaulin covering and be ready for running again the following Spring.


The above text and accompanying photographs are the copyright of the author, Mick Norfolk, and should not be reproduced without prior permission.

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  • 2 months later...

Hi Mick

Just a simply brilliant idea I've just moved house and my layout is going into an insulated outdoor building and I hit on the idea of extending the track it into the garden but it meant building a single track viaduct to span a 16ft lower flower bed to enable it to then loop round on a second raised bed returning back in to the building 'but had no idea off how to do it and my was initial thought was to do exactly as you did and build one out of wood but the maintenance of it was putting me off then bingo I found your article I have two questions for you, how is the track fastened to the blocks and what have you done about wiring to prevent voltage drop. This is my first time on this forum so i'm a complete novice to the garden railway scene.



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Hi Bob and welcome to the forum.

That's really great - I would love to see these viaducts springing up on garden layouts all over the country!

I should perhaps mention the blocks upon which the track is positioned. It wasn't my original intention to use the packing blocks - it was just that I hadn't thought it all through properly beforehand and suddenly realised I was too low, hence the need to raise the level of the track bed slightly. However, thinking about it now the blocks do afford better drainage via the drainage holes and there's never any standing water on the viaduct. The blocks also allow room for wiring to be run and hidden alongside the tracks. The track was temporarily pinned to the blocks using standard track pins before being ballasted using Gaugemaster real stone ballast fixed using floor polish as adhesive. A layer of roofing felt, fastened in place with bitumen adhesive, was laid along the packing blocks as it provides a better surface for the floor polish to adhere to IMO. One year on and everything is as good and firm as it was when originally built.

I've wired all sections of my track, including that across the viaduct, using the busbar method. Two individual cores of insulated wire from a standard 2.5mm cable carry the current around the layout with droppers taken from that to each section of track.

Hope this helps and be sure to keep us informed of your progress.

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  • 7 years later...

Absolutely superb. Again I'm new to all this. First of all how high are these blocks and could they be made to go higher as I'm getting on a bit and my back is not the best. Is it a double track you have and could you paint them if you wanted. Thank you ever so much. Steve

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Hi Steve - thank you for your comments.

I believe the blocks are approx 440mm high standing on end. I wouldn't think they would be quite so sturdy if you were to stand one on top of another but I added additional height to maintain a level track base by building part of my viaduct on top of a raised block wall. Here's that section under construction:


Perhaps something like that would be suitable for you? It's certainly a decent height for accessing easily.

The individual blocks are not quite wide enough for double tracks but that gives you the opportunity to build two viaducts! I don't see any reason why the blocks couldn't be painted and they can be sanded to remove or disguise those horizontal moulding lines.

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Thanks ever so much for getting back to me  Have you done it all the way round the garden like that or have you mixed it up a bit. Are you happy with a single track and what the radius of the curve as it goes round. Sorry about all the question. Thanks a million Steve

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24 minutes ago, surf said:

Thanks ever so much for getting back to me  Have you done it all the way round the garden like that or have you mixed it up a bit. Are you happy with a single track and what the radius of the curve as it goes round. Sorry about all the question. Thanks a million Steve

I'm sure Mick will be back, but I theink he arranged his curves merely to look right. Have you read his posts about his layout? You will get the full story there.


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Take a cab ride around Mick's railway from last year https://www.youtube.com/watch?v=XbcYahEjf3I&t=400s

If you were to ask me to guess I would say the tightest curve around the back of the shed was around 6 foot radius. The scenic areas look shallower, maybe 8ft minimum.

The shallower the curve the better it looks but the more space it will take out of the garden.
What you do really has to be driven by what you have the space for and what obstacles you have in the garden. I went out with a long pole and waved it about to figure what likely radii would work around my garden.

Edited by Clay Mills Junction
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Your a star thanks for all that. I'm going to take my time and try and get it so "it looks right" in the garden and it can become part of the garden. I have basically a square garden no obstructions as such. As somebody alluded to it's more of an engineering job when your planning an outside railway... Cheers Steve

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The above replies sum everything up quite well.

It was always my intention to build the layout predominantly as a single line. At first it was intended to represent a preserved line, which it still could do, but then I began leaning more towards Scottish stock and trying to incorporate a feel for a Highland type single line.

A single line usually dictates a lower line speed which I much prefer. I just think a train trundling slowly around the garden looks so much better than one racing around at high speed so yes, a single line with a realistically slower line speed where a circuit can take 2 minutes or more was my plan from the outset.

I can't recall the exact radius of the curves although it's probably mentioned in my layout thread somewhere but as has already been said it was more to do with them looking right while at the same time trying to keep close to the boundaries of the garden. I did try to avoid overly long straight sections so there's plenty of gentle curves.

Yes I'm still happy with just a single line. There's easily enough operating potential for me but I do occasionally run 2 trains at once either both in the same direction or, with the aid of the short double track/passing loop section, in opposite directions. I do of course have the indoor station sections as well which adds operating potential.

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  • 1 month later...

Hi Barry

On the shorter viaduct, the first one I built, I ended up with it slightly too low to join up with the track inside the shed (measure twice cut once!). So I chose to cut thin slices of aerated block and glued them on top of the blocks, between the parapet walls, to raise the track bed to the correct height. If you look carefully at the photo above of the class 37 and chocolate/cream Mk1 coaches crossing the viaduct you'll see that the loco is on track that's sitting above the tops of the building blocks with correspondingly taller parapet walls. Our little secret!

Yes, I initially glued a strip of roofing felt along the track bed, stuck down with a liberal coating of bitumen adhesive. I'd intended leaving it that way but later on I decided to ballast so simply ballasted over the top. I used exterior 'yacht' type varnish thinned with white spirit to secure the ballast - applied once suitably thinned, with a pipette. SBR bonding compound is often mentioned as a suitable ballast fixative these days but I haven't tried that yet. It's probably easier to use and less messy than the varnish.

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I don't suppose you need the roofing felt Barry it's just that that was how I initially intended leaving it. The blocks themselves can be finished nice and smooth to take track directly but, don't ask me why, I'd still prefer to add a layer of roofing felt if I were doing it again as it just 'feels' better that way.

I used an exterior grab type adhesive to glue the small pieces of block onto the top of the main ones. I'm not sure what brand it was back then but I now use 'Gorilla' brand which seems much more durable. The main blocks I cemented together with mortar - again just because to me that felt the better way of doing it.

You'll need plenty of old saws!

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  • 7 months later...

For anyone in the future building these viaducts. I built half a dozen or so with the saw and rasp method. Then I came across 127mm diamond hole cutters. Gives a perfect arch when you clamp all the blocks together. Unfortunately the best method for cutting the support was a diamond tipped bandsaw blade. I was quoted £150 for that so I bought 2 block saws instead 😂.


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That looks a much easier method than the one I adopted Joe and I'm really pleased to see someone else constructing a viaduct using those aerated blocks. Brings back memories just seeing them lined up like that!

Let us know how you get on and I'd love to see photos of the finished product once it's complete. Oh and if you can come up with a better solution for the parapet walls then let me know as the pigeons have recently flattened some of mine again during their mating rituals!

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  • 2 months later...

I got hold of a Punkt cutter for £10 on eBay, started to wear after 30 or so cuts. You'll know when it starts snagging and your wrists hurt. Not sure if wet cutting helps, but the blocks are like sponges, when they get wet they stay wet. 

Ended up with 50 arches, making them rise at a 4/100 gradient while keeping the arches lined up is best avoided, my brain hurts. The left hand side of the viaduct I gave up through boredom and layed a concrete base on a 4/100 slope into the gazebo instead, then mastic'd them into place on top. Seem solid enough.

I got sheets of OSB, cut loads of 8ft long 4" wide straights. Got the tape and string out to measure 60" radius curves for my tracksetta gauge. It was then simply a case of felting them and screwing them all down. Tracks going down over the weekend, then need to work out how to waterproof the point motors on a passing loop.

Eventually I'm going to put a Wye junction in to get into the back of the garage, probably next year now.





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Oh and don't let kids play with a footballs near them 😭. These were the early ones I wasn't happy with anyway. 

Got hold of some angle iron, and steel lbits and bobs, going to build a suspension bridge of some sort for this section.



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