You should have a good idea what dimensions are required at this stage. If you're modelling a prototype, then research is paramount: if an item on a fictional route, then you should have dimensions required measured from the terrain and layout. If fictional, it's worth keeping the item in context: make it look as if it should be there in real life, made with the correct materials: it'll help the overall believability of the route. Just because a viaduct doesn't have rivets, doesn't mean you can't count them (if you see what I mean).
In this case, it's purely fictional and very, very simple: in reality I would have a more complex structure, and would include an arch ring of stone on each arch: as it is, in real life, it'd fall down.
So, to begin:
A box of the overall size required. Nothing really more to do here, except to note that I've moved it so the box is at the origin (0,0,0): easy to do: select the Move tool, and type zeroes into each of the axis boxes on the status line. There's no real reason to do this, except it keeps things tidy and helps with complex models. Also, to save a few steps later, make sure the Generate Mapping Coords box is checked.
Next, a cylinder: if you generate it in the Left viewport, it'll be in the correct orientation: if not, use the rotate tool and make sure Angle Snap is active on the status line. The cylinder should be the radius of the arch, and should be longer than the width of the box. How much doesn't matter. Make sure the height segments are set at 1 or you'll introduce a lot of polygons, also, again make sure you check the Generate Mapping Coords.
The number of sides is up to you. I find 12 is a reasonable compromise between number of polygons and jagged edges in a viaduct this size, but you my want to increase this; it can depend on the location and texture used, as well.
Check the Slice On box, and set the Slice From and Slice To boxes to result in a half cylinder. At the moment, it doesn't matter where the cylinder is placed relative to the box.
I've moved the half cylinder up to the final height, and it should be approximately central to the width, but not worried about position along the length of the box a the moment.
Go to the modify panel and, with the cylinder selected, drop down and select the Edit Mesh modifier.
Go to Polygon sub-object mode, select the two polygons (four faces) under the cylinder: select Extrude from the Edit Geometry rollout, ensure that the Group Normal box is checked and drag the polys down to below the bottom of the box.
Next: