I was starting to wrap myself around into a tighter and tighter circle over how to determine the maximum dimensions for the module. The circle was getting so tight I was in danger of becoming a personal size black hole.

I decided to regroup and keep it simple. The maximum dimensions and weight for shipping something through the US Post office is (I also checked the Canadian import regulations and their box dimensions are the same):

- Length – 46″
- Width – 35″
- Height – 46″
- Combined Height and Girth – 108″
- Weight 70 pounds

So my module will have to fit within those dimensions, including shipping materials. I guess that means that if it is to be 48″ long, which was my initial intent, it will have to fold, or spindle, or mutilate to fit within the shipping box.

I still can’t talk too much about the standard I am building to since it is still under review but here are the basics:

- Width at interface (including fascias) 12″
- Track centered at interface
- Interface 4″ deep at track
- Height floor to rail head 36″ to 56″, adjustable

So now I have my basic parameters for the module. I just need to design a 48″ x 12″ module with adjustable 36″ to 56″ legs to fit inside a 46″ x 35″ x 46″ box. Actually, smaller than that because if I make the box 46″ long the box will be less than 8″ wide (girth = 2 x length[the longest dimension] + 2 x width) and the box must be no more than 108″ combined height and girth.

This isn’t really any different from what anyone goes through building modules. You have to take into account the interior dimensions of the transport vehicle, and how many people there are going to be to carry it at the least.

*The Bridge*

The centerpoint of the module is supposed to be a Howe Truss bridge over the San Pedro River at Fairbank. Since the bridge is the centerpoint of the module and I don’t know of a good way to make a believable and functional Howe Truss that is not all in one piece, it is the first set of dimensions that need to be finalized.

I have no idea what kind of a bridge crossed the San Pedro near Fairbank and in fact I have not found a reference that says definitively that there even was a bridge near Fairbank. This is where “what-if” comes into its own. I have decided that a railroad bridge does cross the San Pedro at Fairbank. It is a Howe Truss bridge. I haven’t defined its length yet but I have a few parameters that will help decide it.

According to several sources wooden Howe Truss bridges could be built between 50 and 150 feet in length per span. In 1877, the Southern Pacific Railroad crossed 605 feet of the Colorado River at Yuma Arizona by building 7 spans, 6 of 85 feet and 1 of 95 feet. That tells me that in 1877, Southern Pacific engineers considered 100 feet as the outside limits of a Howe Truss span. So as a start my truss spans will be between 50 and 95 feet long each (12.5″ – 23.75″ in O scale).

I also have a width for the spans. Most On30 groups use the NMRA On3 standards for clearance. I will do likewise. The On3 clearance for a section of straight is 1.5″ from the centerline of the track to the nearest obstacle for a total of 3″ clearance side-to-side or 12′ in 1:1 scale. In looking at various sources the narrowest Howe Truss span I have seen is 12′-9″ on a 3′ gauge line. Since I am modeling a 30″ gauge line I can assume that my equipment will be somewhat smaller but I am going to stick with a 12′-9″ inside clearance, or 3-3/16″. My two truss will be 5/16″ thick each. Therefore the total width of my truss spans will be 3-13/16″ or a scale 15′-3″.

The height from the top of the bottom chord to the bottom of the top cord is 23′ or 5-3/4″. My top chord will be 1/4″ or a scale 12″ and my bottom chord will be 5/16″ or a scale 15″. Therefore the total height not including the bridge supports or abuttments will be 6-5/16″ or a scale 25′-3″.

Size of one span:

- Actual: 12.5-23.75″ x 3-13/16″ x 6-5/16″
- Scale: 50-95′ x 15′-3″ x 25’3″

The next obvious question is how wide is the San Pedro River at Fairbank, or more correctly how wide was the San Pedro River at Fairbank in the 1880s? I don’t know, yet. The San Pedro, like many Southwestern rivers did not have clearly defined banks. Sometimes the river is in flood and the area looks like a huge but strangly dry-looking marsh. Sometimes the river is low and whole sections of it are underground only to reappear further down river. According to some sources, in the 1880s the San Pedro was always above ground in the area of Fairbank but that does not help define its width. What I need to do is define a height above the riverbed for the bridge and then see how far back in the floodplain that takes the bridge.

So my next order of business is to research the San Pedro river basin near Fairbank. This will determine the number of spans needed and the length of each span. I may also decide to use trestle approaches and just one or two spans over the “normal” river depending on the width of the floodplain.