Best Material for a Concrete Arch Bridge - Model Railroader Magazine

George,

If it were me building this bridge, I would choose extruded polystyrene building foam (EPS), the pink or blue foam at Lowe's or Home depot.  Lowes has some green stuff that I do not care much for.   I like the pink and blue.     I actually build foam model bridges, and have built many, over the years.    I am an O gauge (and S) primarily.   I realize that your bridge would be roughly half the size of what I might build for the above project, but foam is strong, don't be intimidated by it's light weight.    After a good coat of acrylic paint (don't use oil base, it will eat the foam), your bridge will become very strong, even strong enough to withstand dropping from almost any height.     As to its ability to carry the weight of your trains across the arches, I would expect no problems, although on a span such as the one above, I would probably err on the side of caution since it i s1/2 the size that I am experienced with, therefore I would add a stiffener of sorts (perhaps a T imbedded in the deck for the length of the bridge, that would be concealed after laying track.    As to the workability of the extruded foam, it is very workable.    You can sand it, fill it with caulk where necessary, carve it, melt it, etc.    It accepts paint very nicely, and can provide you with something that is very deceiving to the eye, as it really can take on the appearance of real concrete or stone.    

Hope this information helps you in some way.

best regards,

Tim