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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.


Well-Known Member
Thomas Viaduct


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 commisioned and built so early on by B&O's first president,...and its still in use TODAY !!



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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.

Very rough mock-up at the beginning,...those blocks of wood are not the correct height

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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Well-Known Member
Modern day plug making
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?​
Two arch shapes I have in mind,..​
The viaduct frame,..​
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Well-Known Member
close to the walls & size of arches

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.


Well-Known Member
Looks interesting! I've done something similar in the past, only using tunnel portals to form the arches.


Well-Known Member
Yesterday evening I experimented with making the arches all identical rather than 2 sizes (with the exception of that one single one over track coming thru the back wall).

I think it looks presentable, but I do have a question about the very lower portion where the footings are getting to be quite a bit 'wider' compared to the others ?.....opinions??

BTW, in constructing this mock-up I had to place each 6" wide arch drawing at different heights to the adjacency ones,...and even more exaggerated so in this particular higher grade slope (about 4%). I imagine this means I would have to do the same offset of each 6" piece whether I was constructing this 'wall of arches' with a casting method, or a decorative flexible facing material ??


Same Ol' Buzzard
I was visiting a friend this past Sat, and he made mention of a German fellow who made very nice viaduct moldings. He gave me this reference.

Good photos,...wonder if there is a method to translate this PDF into english?
Yes there is. This is how it works in Firefox.
Open the PDF.
Download it to your computer, it should end up in your download folder.
Open Google translate in a different tab.
Click on Documents, then click on Browse Computer. Find the PDF in your download folder and click (open) on it. It's named "BA_Lennebruecke".
In Google Translate, click on "translate".
It may take a few seconds and you will not get the pictures but you will get the translation.

There may be an easier way, but I don't know it.


Well-Known Member
Also, the width at the base of each where each arch meets should be the same width, even though the arch is getting shallower.
Glad you brought this subject up, so I had a new look at the situation today. Turns out I CAN made those base dimensions more uniform,..simply by the way I cut and paste the master image. I did this quick little experiment today when I cut the original 6" wide image down to 5" wide for those shorter arches in the lower portion of the viaduct. This cut-and-paste cardstock method allows for such possibilities,...that would have been much more involved if I were using plaster moldings.

Lets see if my crude photos show the difference.

original wider bases

narrower bases on those last 4

BTW, that's the famous clock tower in downtown Baltimore (background to be there in that corner). Going to have to make a cardstock image of that as well.


Well-Known Member
Substructure of Single Track Viaduct Portion

Couple of days ago I finished the substructure of that portion of the viaduct that supports the single track that sits behind the turntable area and in front of Balt. The roadbed and the vertical supports are all constructed of that cellular PVC material I've spoken of,...and glued together with PVC cement.

In this photo I picked the whole structure up and moved it out to my carport work bench for some final tweaking, and tapering down that end that reaches ground level.

Here is a little close up of some scraps of that PVC material I used.

BTW, having a chop saw was really handy in getting those very straight cuts,...and a piece of 180 sandpaper lying flat on the table top made cleaning any flash a breeze.

Now I can get on with a the final accurate location of my turntable and its roundhouse,...and subsequently the trackage associated with it. I was going to go ahead with cutting the big hole in the 3/4"plywood deck for the turntable, but my friend suggested wisely that I wait to do that outside the shed, as the router cutting that big hole would make quite a mess.


Well-Known Member
Finalize Subframe Structure of Double Track Portion of Viaduct
(and perform open heart surgery....ha...ha)

I went back over to the double track portion of my viaduct to finalize and tweak its vertical support piers.

I have this long piece of stiff aluminum 'door sill' that I have been using to get the grades smooth,

I was under the impression that I had the upper portion of that viaduct done already. ....But then I discovered I needed to provide slightly more clearance under the bridge portion to clear double-stacks coming into port on that track passing underneath. Plus, I discovered that I need to re-level my basic plywood down that side of the layout (long explanation deleted). So all my previous measurements, and the already cut support piers were going to have to be replaced or recut to new dimensions.

As if that wasn't enough of a disruption I discovered another problem affecting the viaduct on that side. It turns out I have two sheets of plywood deck meeting in that area,..and forming a corner piece.
(hidden under that paper template)

I was going to have to perform surgery ( I kiddingly referred to it as performing open heart surgery),...cut back the covering paper template, roll it up, so I could access the innards.

I put a 'backing plate' of ¾” plywood underneath such that it got glued and screwed to both deck pieces of deck material, plus it got an extra corner piece glued and screwed in as well.

That is wax paper hanging there to keep from gluing all this mess to the metal bench work

Now I can get on with the FINAL cutting and fitting of the viaduct frame.
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Well-Known Member
As the viaduct approaches deck level, the arch structure is no longer utilized,...just stone sides, no arches.

(there is that alum grade tool again)

Turns out there is not much need for visible arches here anyway, as they would likely get lost behind some of the buildings I might place along here,..for instances



Loveably weird
Looking good! I know what you mean about the plywood seams. I did the same on my layout: 1/4" backing plate.


Well-Known Member
I have some nice stiff alum plate material I am going to utilize at most of my plywood joints, but for this particular spot I though the plywood backing plate a better option. I had considered using biscuit joints, but I was not so willing to join (glue) all my individual deck edges into really big one-piece entities.


Well-Known Member

Yesterday I was asking for help to get some of these arch images to a correct size, and some of them at a reduced size as they descend the grade. I needed to determine how many, and how small these arches needed to be, I could go back and redo those vertical supports of the frame structure.

I've now chosen to utilize 6 inches wide for the upper arches, then two 5.5 inchers, then two 5 inchers, and finally two 4.5 inchers.

I also determined that I want to utilize that second choice of a 'image master' that emerged after I first did these mock-ups,...this image

I also like the stone post sticking up from the roadbed, the prototype viaduct.

In this view I've cut out all the interiors of the arches.

And here you can see a few of those vertical supports that need moving

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