Ballast Deck Girder Model Railroad Bridges

This is a set of detailed girder bridge parametric models created with OpenSCAD that can be customized by changing the length and other details, and new styles can reuse the code. It started out with a design that is common on the Michigan Central railroad line along the Huron River, which it crosses frequently.  The bridge next to the Barton Dam is a little unique because the stiffeners are not spaced uniformly along the girder giving it a “gaped tooth” look.  Amtrak's Wolverine line is still using this track, along with a few Norfolk Southern freights.  The New York Central RR appears to really like the ballast deck approach, and I saw similar bridges at the NYC National Museum in Elkhart, Indiana. I could not find any commercial kits of this style and thought it would be an interesting project to use to learn OpenSCAD.  

Since I first posted these models in 2022, OpenSCAD has been greatly improved in its "development" version. Because of the many facets in the riveted construction (literally a million facets in most of these models) the stable version of OpenSCAD takes hours to generate a rendering. As of 2026 the development version can render these models in a few seconds and is very stable. It also has some features that enabled me to fix some of the problems with the original models. I have updated all of the files, improved the rounded ends girders a lot, and separated the different modules into separate files to make maintaining the OpenSCAD code easier.  OpenSCAD is free, and if you go to the download page and scroll down you can find the development version. The devs are working to release the new version, but it is a hobby project (like this one) so it may still be a while. Use the "Customizer" to change bridge length and width, break the pieces up for printing, etc.

The biggest concern about printing this type of model is the thin size of the details, particularly beam ends and the protruding parts of angles.   I have an option for FFFprinting, which if true increases the size of the thinnest sections.  The other concern is the surface finish, so I printed each girder in two pieces and used ironing.

I have included the 3mf of the Barton Dam Bridge so you can see the print settings I used.  Note that the girders need to be printed twice, the sliding anchor types need to be printed four times, the center anchor twice, and the half x brace 16 times for the two spans to match the prototype.  Also dowels are included to keep the pieces in alignment, 18 are needed.

Assembly of the pieces is a challenge because they are somewhat fragile and do not have flat clamping surfaces.  The stl file includes clamping blocks with appropriate labels.  

I used Super Clamp with customizer, by Erick Neilson of Vancouver B.C., 100mm tall and 60mm deep, which is a remake of DIY Clamp (customizable) by Jes jj of Rivne, Ukraine from Thingiverse-

https://www.thingiverse.com/thing:3075868

This clamp is able to stand up on a table by itself, which is nice because its hard to get the blocks into place and avoid smashing the details.  If its too hard to fidget with all the blocks, some two sided tape probably could be used to keep them in place.  I sanded the faying surfaces and used a very thin layer of slow setting epoxy.  For the prototype I used two clamps, but four would have been better. Not much pressure is needed, just enough to squeeze out the excess glue. Cleaning off the excess epoxy immediately seemed to be the best approach.

First glue each girder together, the two sides are identical.  The prototype is a two span bridge, so a total of four girders are required.  Dowels needed to be sanded and are quite small, but are a big help to keep it straight.  Also glue two x brace halfs together to form 8 x braces.

Next glue each pair of girders together and glue the x braces inside to a stiffener.  I just dabbed epoxy onto the four gussets on one side and pressed it gently onto the stiffener. 

The Argo Dam bridge is an open deck style.  Here is a link to photos of this bridge:

https://historicbridges.org/bridges/browser/photosviewer.php?bridgebrowser=other/annarbor/&gallerynum=1&gallerysize=3

Also included is a Wabash Railroad, then Norfolk & Western, and now Norfok Southern bridge in Milan, Michigan over US 23 built in 1950, with notable architectural treatment of approach and abutment.  I used super glue and no clamps, after sanding faying surfaces with 400 grit sandpaper.  It went together fast, with no clamping required.  Abutment face is best printed on end, as shown in 3mf file.  It meets the NMRA standard for classic period if the track is raised slightly (such as on cork, which should be done since its a ballast deck through girder bridge).  

https://historicbridges.org/bridges/browser/?bridgebrowser=michigan/us23rr/

The Schuylkill River Bridge is a composite steel girder and concrete beam bridge, and is part of a longer concrete bridge that I have posted in a different model.

I have included one stl of each of the bridges assembled so you can see what they look like and one broken up for printing. The OpenSCAD programs are all in the OpenSCAD.zip file, since individual scad files are not supported.

This is still a "work in progress", and I have not printed or tested all of the ranges of different parameters. So please post any comments or questions. I would appreciate if anyone who prints a model would post their results.