My question is what wood is best for bending and how to do it without steam, the bridge is in n scale.
A photo of the bridge I’m modeling.
Thin basswood sheet; you rip multiple parallel kerfs to a depth close to the face, and the wood will then bend easily, Make a mandrel the size of the arch, bend the wood over it ‘kerfs out’ and tie or pin it in position, and fill the kerfs with a material that dries strong with minimal shrinkage.
Is there a reason you’re using wood? In N scale, bending wood may be difficult.
Have you considered using thin styrene? It will bend to this shape, and it can be made to look like the stone.
the two RR viaducts i know of in New Brunswick, NJ and Cumberland, MD are concrete
The arches of this double track HO scale viaduct are formed from card.
Underneath the viaduct. by Bear, on Flickr
I’d suggest in most cases, and the above is one, that the underneath is not readily viewed, anyway.
Cheers, the Bear. 
Unfortunately I don’t have pictures of this bridge under construction.
The long sides started as 1/4” plywood. The arches were cut out and the sides were attached to a board serving as the roadway with small pieces of the same board at the bottom to maintain the shape. I then glued cardstock inside the arches to close it up. Sculptamold was then spread over the entire structure and the stonework was carved. It’s been in place for 28 years.
Edit: After thinking about it I might have used 1/8” plywood.
Like cardboard?
I don’t remember the exact size but I bought a piece of Woodland Scenics expanded polystyrene foam; it may have been 12x24x2 but I’m not sure.
The next step was to cut two posterboard arches that were exactly the same size. I thin pinned these on opposite sides of the foam block. I took my wire foam curter and simply cut out the foam that was inside of the arch templates. While I left mine that way, if you wanted, you could easily carve any amount of additional detail into the foam that you might want.
Gidday Jake, it depends on definition, and location because I tend to regard cardboard as being corrugated, where as card is “thick paper” and more flexible.
Why I mentioned location is that we use GSM (grams per square metre) as the means of determining paper thickness, whereas it appears the US uses lbs.
It’s a long time ago since I built the viaduct and have no idea what thickness I use, but if my research is correct, you should be able to use any thickness between 80 lbs (216 gsm) and 110 lbs (293 gsm).and get a satisfactory result.
I guess as a common denominator and while it may be simplistic, a pack of playing cards, is an example of the material I refer to as card.
Hope I haven’t muddied the waters too much.
Cheers, the Bear.
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In the United Ststes, the former is referred to as ‘corrugated cardboard’ (and in the Northeast at least, a box made from it would be called a corrugated box as often as a ‘cardboard box’), and the latter as ‘cardstock’, with a nominal ‘weight’ higher than that for sheet paper.
A third material was known as ‘shirt cardboard’ and was heavier gauge than cardstock, gray and rough and heavier than cardstock. This was used to stiffen button-down shirts from the dry cleaners if you ordered them ‘box, no starch’’ instead of ‘hanger’. This was a fine material for building structures that would be faced with paper, but it needed to be moistened and then carefully clamped in place to dry if you wanted to bend it.
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