On the first page I calculated that it would require 3ea 4-1/2″x12″ stringers under each On30 rail. I’m happy with just 2ea under the crane rail.
The following is from A treatise on wooden trestle bridges By Wolcott Cronk Foster
Stringers.—A stringer should be placed immediately beneath each rail, and in order to guard against defective timber it ought to be “split” or composed of two or more pieces. These pieces should be separated from each other by either cast-iron washers or spools, or wooden packing-blocks, or both. A considerable difference exists in the present practice as to the amount of separation. It varies all the way from nothing to 13 in. From 1-1/2 in. to 2 in. is a very good distance.
Wooden packing-blocks – These are to be placed immediately above the caps. Those packing-blocks which are notched are of course placed so that the cap fits into the notch. (among those shown I picked three – fig 73-75. The book indicates that the general form of Fig. 75 was preferred.)
I underlined wooden packing-blocks above since for modeling purposes, cast-iron washers or spools would be a major pain to model. I much prefer the wooden packing-blocks. I also underlined the 2 in. dimension since I have some wooden coffee stirrers that fit this requirement exactly being .040″x.125″.
A quick look at the wooden packing-blocks shown and we can get some lengths – Fig 73 is 8ft; Fig 74 5ft-3in and Fig 75 6ft. The book preferred Fig 75. The bolt pattern perhaps. I will use that bolt pattern and length of Fig 75 but use the shape of Fig 73.
This is important for the model in that the bolt heads will show on the outside of the stringer.
Frequently the packing-blocks made of the heavier material are used merely as splice-blocks
Many fasten the stringers together by intermediate bolts placed either at the centre of the span or at regular intervals along it. Separators or packing-blocks are of course required to be placed between the stringers wherever these bolts are located. With good timber and spans of 12 ft. to 14 ft. these intermediate bolts are not necessary, and may be just as well omitted as not.
When it is possible, the stringer-pieces should be long enough to extend over two spans and the joints broken. The stringer-joints shown in Fig. 83 is to be greatly pre ferred, because, should the support for any reason become weakened, the joint, when it settles as a weight comes upon it, closes at the top and tends to open at the bottom. Now the lower bolts act somewhat as a fulcrum, and the effect will be to tend toward splitting the stringer from these bolts to the nearest end. As this arrangement gives the most material where there is the greatest liability to split, and consequently at the weakest point, it forms the strongest kind of a joint.
The bolts holding the stringer-pieces together, and which are called packing-bolts, should be long enough to extend clear through from face to face of the complete stringer, and allow of placing a cast-iron washer under both nut and head.
Packing-Block Design : I had no problem modifying the Packing-Block above to fit my stringers. On the left I have three variants – the one on the bottom makes sense in that it allows water to run off. I am tending toward the simpler version at the top.
Update: I ended up going with the one in the center. Mocking it up in Sketchup I didn’t like the way the top version looked from underneath. Mucking about with trying to taper the top though like the bottom version .. rain friendly or not .. seemed to me to be a little much since so little of it would be seen from the top in the space between the ties .. and .. a flat top was probably more common then not with the railroads.
Assembling : The problem is .. I’m not 1:48. Here’s the problem as I see it. To use the Packing-Block as shown it has to be notched over the cap. The problem is .. how to do that?