Stone Arch Viaduct / Bridge
I've included both names as I have seen folks address them with either name. I think what I am trying to create is principally a viaduct, but yesterday I inserted a bridge in a portion of it. I did it in a straight track portion, so I could alternately allow for it to be included in the final trackwork, or perhaps not.

The bottom deck of my layout is to represent principally the city Baltimore, home of America's start with the railroad industry. There is a neat stone arch bridge there in the suburbs, the Thomas viaduct that is even more famous as I read more about it. I wanted to include such a viaduct on my layout as a landmark representative of Baltimore.

I recently became aware that I had several photos I had posted of this viaduct that were not actually the Thomas viaduct, even thought they look very similar. One was a double track affair, the Thomas one, and one was a single track affair located in another state. I'll get into that later.

Turns out I have need of both a single track portion and a double track portion. How to construct them is another big question mark for me. I am making a mock-up using foamcore board right now, and it is presenting even more challenges than I originally anticipated.

Quote:The Thomas Viaduct spans the Patapsco River and Patapsco Valley between Relay, Maryland and Elkridge, Maryland, USA. It was commissioned by the Baltimore and Ohio Railroad (B&O); built between July 4, 1833, and July 4, 1835; and named for Philip E. Thomas, the company's first president.[3]
At its completion, the Thomas Viaduct was the largest railroad bridge in the United States[4] and the country's first multi-span masonry railroad bridge to be built on a curve. It remains the world's oldest multiple arched stone railroad bridge.[5] In 1964, it was designated as a National Historic Landmark.
Interestingly, it was commissioned and built so early on by B&O's first president,...and its still in use TODAY !!
[Image: 800px-Thomas-viaduct-1.jpg]
18 other photos
...and one of a single arch that I will try to get somewhat correct depending on my layouts geometry.
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The main design problem to overcome was that of constructing such a large bridge on a curve. The design called for several variations in span and  pier widths between the opposite sides of the structure. This problem was solved by having the lateral pier faces laid out
Here are the two zones of my viaduct marked out with the yellow marks. On the right side of the dwg is the double track (2) mainlines leading up most of that side from its start down in the lower end, up to the tunnel entrance to the helix at the back wall.

The single track portion is represented by that track that that splits off of one of the double track mainlines just prior to entering the helix portal and travels along the back wall to curve down and eventually join back in with the inboard mainline down the left side of the layout. This is my 'alternative track' that allows a trains to run continuous loops around the lower level of the layout without having to climb the helix each time.

In both cases the viaduct(s) climb over two different tracks at their upper heights,...near the helix portal.
The longer double track portion climbs a 2.64% grade,... best I can do at the moment.

I'm not even going to mention the grade for the shorter single track portion at this time, as I am sure I will get many naysayers. Lets just say that I envision that in MOST cases the trains will be running down this grade, not up it.
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Very rough mock-up at the beginning,...those blocks of wood are not the correct height
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And those sheets of foamcore standing vertical are no where near correct,...just spare pieces thrown up to approx the outer reaches of the Balt city scene. And that viaduct there is an older plastic one I happen to have laying around.
(I was originally planning on letting the Balt property line come out and cover that curved track exiting/entering the back wall. but there were some problems there with incorporating the viaduct directly into that 'property line' with the city, so I thought it best to put the city line in back of that curved track. I think I will now let that curved track behind the viaduct be one that is exciting/entering the 'harbor tunnel' of Balt)
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Ran into a problem with sizing the arches such that one of those tracks (at least the one that passes at an angle other than 90 degrees) could run underneath. I happen to have a dbl-track curved cord bridge that I placed over that track. May have to utilize that to span some distance between 2 segments of the viaduct. 

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Have you considered using the pink insulation Styrofoam?
It would make it a lot easier to draw out and make.
~~ Mikey KB3VBR (Admin)
~~ NARA Member # 75    
~~ Baldwin Eddystone Unofficial Website

~~ I wonder what that would look like in 1:20.3???
I agree with Mikey. I have one that I made from the pink foam and coated with plaster and stone molds. It is yet to be finished (and may never be done) but that is the way I went. 
Mine is a straight 4 track, nothing near what you are attempting. I will look forward to seeing this worthwhile project completed. 
I considered the 'pink foam' construction methods, but I keep running into the idea of having to carve all those stones details into the surfaces, plus the non-reproducibility of all those multiple arches.

This foam-core mock-up was originally going to be just that,...a mock-up. But I have discovered that this particular foam-core product is much more deserving of being the actual structural portion of the finished product,....then cosmetic skins added to this backbone structure.
My foam-core is actually expanded foam PVC sheet material, very easy to fashion/cut with standard wood working tools, very durable, totally waterproof, etc.
I started out using some 1/4” thick material for the roadbed. Then I experimented around with the arch dimensions. I ended up with 2 arch dimensions,...a 6” inch center-to-center column dimension with a 5” dia arch,...and a 5” cen-to-cen one with a 4” dia arch
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….the larger arch for the double track portion and the smaller arch for the single track portion.

In order to make those arches line up I spaced them along the inboard edge of the rising bridge roadbed,...every 6'' for the double track portion. So now I knew where my risers needed to be,...behind the vertical columns of those arches. I cut these risers out of 1/2” PVC foam,..the same type of material I had on hand for the roadbed. AND then I doubled up the thickness of the risers by gluing two side-by-side making them 1” thick risers...overkill, but I will explain my reasoning later.

Now I had to get the height of each of those multiple risers correct. I measured my ultimate rise, my minimum rise. And the curving lineal distance along the roadbed between those 2 points. I plotted those two heights on the edge of my plywood work table and drew a straight line between them. Now I could go to any point along that line and measure what height riser I needed.
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(BTW, my plywood work table was NOT long enough to get the whole distance in there, so I had to add a temp alum beam to the edge of that table to get an extra 40 + inches in there).

I am going to make all of the risers the same width as my roadbed, 5”, so I cut a 5” wide strip of 1/2” PVC foam, then proceeded to cut off the different height risers. Turns out I need to cut 2 pieces for each riser location so I could double them up to get my 1” thick riser.
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You could shape the bridge and use polystyrene sheets for the stone work These are 7.5x 12 inches 
and a better deal here but they are cardboard.

I was just looking thru a few of those decorative facings and trying to determine:
1) the one that best matches the Thomas viaduct
2) the thickness & durability of the different ones
I like the simplicity of this approach,..

Just need to find a good existing pattern.

Or I wonder in this modern day if such a 'skin covering' could be created by some photographic/printing process like they use to create backgound scenes?
For the most part my arch bridges are going to be fairly close to their back walls,...and/or their backsides are not going to be viewable. So it was my intention to leave off the decorative facings on the back sides.

The two different size arches came about as a result of the spacing of those 2 tracks passing underneath, and the bridge in that one section. Turns out that bridge is 18" long, so divided by 3 it came out to 6" each arch span. I then made the manila paper cut out and it looked good.

When I extended that on towards where the single track curved off to the left and crossed that other track the continued 6" spacing did not work out correctly. So I made the smaller arch at 5". Wala, it worked out just fine. So I just chose to make the single track section all in 5" arches.

I had wanted to limit the number of different size arches, thinking I might end up making duplicate moldings, or duplicate somethings. At the moment I think I have that with only 2 different sizes,...and their legs can be shortened as the grade drops in elevation.

Wonder if I can get someone to create a nice 'master plug' of those 2 arches, that I could take and mold duplicates of?   The vertical joint lines would be covered by a third molding of that proud portion on the columns in the original bridge.
Check out the 2 photos HERE
I'm a total novice at this parts creation technology utilizing computers, laser, etc, please excuse my very limited knowledge with this inquiry.

I'm building a stone arch viaduct that hopefully looks something like the famous Thomas viaduct in Balt. I have the backbone structure of it made from expanded PVC foam. My thoughts are to create the decorative 'skins' in some sort of molded plaster or another product. I would need to create a 'plug' to make the mold that would then create multiple thin arch duplicates.

So my question is, could a photo such as this be used to program a laser cutter to build several plugs for the molds to create the multiple arches?
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Two arch shapes I have in mind,..
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The viaduct frame,..
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It had been pointed out that my first arch shapes I first sketched up where not correct, semi circular rather than elliptical in shape.

So I decided to print out copies of the original photos and create these paper arches.

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Then mount them in a row along the edges of the roadbed,...
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Now if I had a relatively thin piece of plastic or relatively thin vinyl strip that was embossed with the proper stone image, I could just glue that to the edge of the roadbed foam board. Or if I had individual arches made of either this plastic/vinyl/or even hydrocal castings, I could line them up along that outer edge of the roadbed.
Here is an article about making molds and building a stone arch bridge. Dr Wayne swears by Durobond for castings and I heartily agree, though it gets very hard and then is hard to work with. available at any builders supply stores and in 3 setting times 20, 45, and 90 minutes
I really admire the research you have done on this and I do look forward to seeing the project when it is completed. 
(08-31-2019, 12:48 PM)Charlie B Wrote: Here is an article about making molds and building a stone arch bridge. Dr Wayne swears by Durobond for castings and I heartily agree, though it gets very hard and then is hard to work with. available at any builders supply stores and in 3 setting times 20, 45, and 90 minutes
I really admire the research you have done on this and I do look forward to seeing the project when it is completed. 

Thanks for that submission Charlie.

I wonder how that Durobond stands up if molded in a very thin form,...say 1/8" or even 1/16" ?   I  think the other model molding materials will be to brittle at this thickness without some sort of wire screening, etc as reinforcement.

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