I freely admit I stole the first part of this from the DRRR Pier article. Since it was from myself I figure that’s ok.

Stringers. Packing-Blocks and Friends

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.” 1

(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 (the largest of those spools measures about 4″ dia .. in O scale that would only be 0.083″ dia). I much prefer the wooden packing-blocks. I also noted the 2 in. dimension since I had some wooden coffee stirrers that fit this requirement exactly being .040″x.125″.

This was originally written some years back. I have since acquired a resin printer (Prusa SL1) and now have the capability to possibly print the spools. Whether I can or not is still to be determined. My friend Shawn Branstetter wants some for a bridge he is building so I am looking into that. Like I pointed out to Shawn creating these is both simple and complicated (depending on how you squint and it I suppose.)


With this drawing above of the spools it becomes indefinitely easier to get started as we have the dimensions shown .. middle of the page is a 12″ ruler with FIGS. 62 to 72 – Cast-Iron Separators below it. I then imported the drawing into Sketchup where I can instantly scale the drawing using the Tape Measure Tool. Once this is done I had a full-size drawing .. scaled .. for all of the separators. I could then re-scale to whatever modeling scale I wanted. In this case it is US O scale .. 1:48

On a side note. People get twisted into knots trying to scale a model to a specific scale. There is no real reason for that .. at least working in Sketchup as I am doing. I simply made a square next to the drawing 1″ x 1″. I am you see .. still in full-size. Now .. you DO have to use a calculator but it isn’t that hard.

Scale \ inches = \frac{1}{48} (1) = 0.0208"

When working on these small models in Sketchup I work in mm .. it is much easier to work in mm than imperial. Sorry .. but is true. There are other reasons including that I can export a STL file for my slicer and it ‘expects’ mm. So the next step is to convert that 0.0208″ to mm. I do this by multiplying by 25.4

mm = (0.0208)(25.4) = 0.529 \ mm

So at this point we have determined that an inch in US O scale (1:48) is 0.529 – so we use the Tape Measure tool to re-size that 1″ square to 0.529 – and our drawing is now re-sized to O scale mm. With this done we simply draw on top – trace if you will – the old illustration to bring the spacers to life.


I am going to take a small intermission from discussing the modeling to talk about my chat room. I have a chat room on Whereby titled “Deep River Railroad” .. the great thing about this video chat room is that you can share your screen. This means that while I was talking with Shawn I could show him the modeling real time on my Sketchup .. or .. flip over to my website and also a book on Google books .. all the time he sees what I see, he sees the cursor as I point to something. This makes a joint “modeling session” so much better.

So when I am saying something like “Shawn liked No. 66 ..” .. I mean real time with him in a corner of the chat window, me in another and us both looking at my desktop where we are looking at the drawings of various spacers. So. Back to the spacers ..

No. 66

Shawn liked this one so I dimensioned it. I used the circle tool in Sketchup to draw three concentric circles for the shaft hole, hub and outside diameter of the spacer. I then extended those lines over to the side view.

Note: the numbers you see are at best ‘nominal’. This was a drawing (which may or may not have been exact) to begin with. The book it was taken from, published “A Treatise on Wooden Trestle Bridges: According to the Present Practice on American Railroads” by Wolcott C. Foster was published in 1897. The scan of the page can only be enlarged so far – between the print resolution of the book and whatever resolution of the scan we can only approximate with our tracing. The numbers though do give us a starting point as the original would have been in inches and even numbers or fractions.

I show both the dimensions in millimetres (O scaled) and inches as calculated back from that to get our approximation of the original sizes (this is mostly for our amusement as the  actual model dimensions will start from the O scale millimetres and go from there.

  1. A Treatise on Wooden Trestle Bridges According to the Present Practice on American Railroads, by Wolcott C. Poster – Page 43[]

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