Some really good ideas coming out here.
I made a viaduct which was about 41/2 feet long, so perhaps a little smaller than you're thinking of!
My construction technique was to use 2mm MDF for the sides and inner faces of the arches as it curves nicely and can be partly cut through on the reverse side to further improve curvature. The sides and track bed were all braced by using soft wood internal braces. Such as 2" x 1" or 3" x 1" timber. Once the basic shape was contoured the while thing was covered in Superquick brick paper, as this was what I had to hand at the time. The bridge looked great when viewed from a distance. Brick paper is ok, but a better effect can be obtained by using embossed card (Cardboard or Platicard) which would improve the look at close quarters. Alternatively, the sides etc can be covered in fine modelling clay (Das for example) available from most Art shops and while its still damp carved into a stone block effect with the aid of a pointed tool (Ground down old screwdriver for example).
If you choose to use MDF I would recommend coating the base model with varnish all over to protect the MDF from any ingress of damp which could 'blow out' the MDF sheets. MDF is readily available form most good DIY stores.
I made a viaduct which was about 41/2 feet long, so perhaps a little smaller than you're thinking of!
My construction technique was to use 2mm MDF for the sides and inner faces of the arches as it curves nicely and can be partly cut through on the reverse side to further improve curvature. The sides and track bed were all braced by using soft wood internal braces. Such as 2" x 1" or 3" x 1" timber. Once the basic shape was contoured the while thing was covered in Superquick brick paper, as this was what I had to hand at the time. The bridge looked great when viewed from a distance. Brick paper is ok, but a better effect can be obtained by using embossed card (Cardboard or Platicard) which would improve the look at close quarters. Alternatively, the sides etc can be covered in fine modelling clay (Das for example) available from most Art shops and while its still damp carved into a stone block effect with the aid of a pointed tool (Ground down old screwdriver for example).
If you choose to use MDF I would recommend coating the base model with varnish all over to protect the MDF from any ingress of damp which could 'blow out' the MDF sheets. MDF is readily available form most good DIY stores.
