SP Jawbone Branch (Part 37) - Placement of NG Trestle & 1st Curve

Continuing the construction blog of Owenyo - Module #3 (western section of Owenyo) on my Jawbone Branch layout.  It's time to get down to brass tacks with selective compression and positioning of two main elements at the west end of Owenyo.

SP Owenyo High Line Trestle - Alden Armstrong photo Cropped

This great detail color photo from Alden Armstrong shows the red fire tool boxes and barrels from the South and NG side of the trestle.  This will be the side in the corner on the HO model.

Main Curve Alignment

The main curve on Owenyo #3 Module is planned to be 48" Radius with at least 1/4" spiral easements.  This will not be the easiest curve with easements to lay out to start with.  The west switch of Owenyo will actually be right on top of the spiral easement at the east end of the curve.  The west end of the curve will have the switch to the transfer trestle right on top of it's easement as well, and a joint across the closer rails of the switch.  The module will be transitioning from flat at the east end to a 1% grade by the west end and there's going to be a small 12ft ballast-deck bridge just west of the siding switch. All of these issues add to the complexity of this section of benchwork.

Tripod with cellphone holder to form the pivot to swing my curve.

I didn't have room in the layout space to fully set the center point for the curve and then swing it before cutting the material into the rough shape for the module.  Now that it's clamped in place, I can set up a tripod to be the center point.  

Closeup of the clamp and wood blocks to make this work.

I used the camera mount for my cellphone, clamping a 49-odd inch long 2x2 into it and using a couple scraps to make up the difference in bite size.  A small hobby clamp was used to tighten the grip of the cell phone holder.

Working out offset from Centerlines of the main Owenyo Tangent.

Starting at the east end of the curve at the West House Switch of Owenyo (SW5227, for those that are following along with switch numbers).  I have started finding some issues with track spacing changing by the west end of Owenyo, so there's over 1/8" indicated error.  The offset I have sketched in here is about 3/8", which puts the adjusted offset somewhere between 1/4" and 1/2".  This will be better to have a larger offset as the SW 5227 is going to be in the middle of the transition, which will require a straight section through the frog.  The transition will be at least 12", possibly as much as 18" if I'm pushing towards 1/2" offset.  At this point I'm just working with tangents, arcs, and offsets.  I'll cover the marking of the easement centerlines later once I get the module's structure done and closer to laying track.  All I have to do now is account for the wiggle-room to build the track with the easements.

West end of the curve at SW5226 to the NG "Highline" Trestle.

Cross-check myself at the west end at the Trestle Switch (SW5226).  This took about three tries to cross check the position and get a 1/4" offset at the west end and maintain the 1/4-1/2" offset at the east end by slightly adjusting the position of the tripod in the room.

Swinging the arc with my mechanical pencil placed right against the end of the 2x2 arc board.

By this point I've already transferred the 25.5" radius for the trestle CL curve and offset arcs for the edges of the roadbed to be cut next.

Completed swing of 48" arc with Caboose Industries ground throws in rough position.

Now that the centerlines are marked within 1/8", which is good enough for now.  Let's move on to looking at the other major element of this module, the Narrow Gauge "Highline" Transfer Trestle.

Trestle Position, Size, & Placement

SPNG shoving the Dump Trestle with five+ cars - Andrew Brandon collection

In this action shot from Andrew Brandon's collection, I can get a much better idea of the spacing and details of the trestle.  Notice the three closely spaced bents at the transition from the grade to level unloading section of the trestle.  These can be picked out in the remains shown in the Google Maps photos (second below).

Owenyo From Transfer Trestle Brandon Coll WM

Also the small wash remains, which had a 12ft trestle over it, are still showing with the 2021 imagery.  This wash on the prototype went under the grade section of the trestle.  From the aerial/satalite photo below, we can see that it extended under the trestle and NG to a white sandy basin at the far left of the photo.

Goggle Maps, North to Left, scaled for 50ft

Measuring the length from the six footings (transition point) to the beginning of the curve (just right of center in view above) shows about 200 feet, or four 45ft coupled car lengths.  So my original plan to have the loading section of the trestle being five cars long is actually wrong from the evidence now that I look closer.  Shortening my trestle transfer section to four cars will make it fit in the space better and I'll be able to model more of the transition section before the model hits the wall.

Trestle Position?

Checking my closest approach offset.  Tip of ruler is 21ft offset from planned 48" radius CL.

Looking at the prototype photo standing on the transition section of the trestle looking into Owenyo, shows the gauge of the SG curve and from that an estimate can be made to how close the centerline of the trestle grade approach is.  Standard Gauge is 56.5".  So I measured the gauge at Bridge 522.82 as 0.440", measuring from the far rail to where the boards that the photographer is standing is 2.26".  Scaling that ratio is 5.136..." and multiplying that by the gauge of 56.5", which is 290.2", converting to feet is 24.18feet.  So I need about 24-25 feet centerline to centerline.

Mockup of trestle with SG cars under the trestle and NG on top.

The Trestle position is roughly outlined by the paper template.  This position is nailed down because of the 25.5" radius coming along the wall and joining over at the west end of the curve (at right), combined with the four car lengths of straight track that is the trestle's transfer loading area.

Bridge Position?

Two possible positions for Bridge 522.82 based on curve position from SW5227.

The compression on the model will still force me to put the bridge into the main SG pit section of the model, because I'll be shifting the trestle about 130 feet to the north so that I can have the loading section of the trestle modeled and still fit on the module.

Looking west to see how the scene comes together.

Keeping in mind my engineering requirements and hydrodynamics... if I can have extra compression between the west switch (SW5227) at Owenyo and the bridge, I might be able to have the wash go under the transition section of the trestle, and not 'drowned' the SG pit when it rains at Owenyo.  Compared to these photos, I may push Bridge 522.82 almost all the way back to the switch stand of SW 5227.

Next Steps & Closing

SP 2335 & 2350 arrive at Owenyo with excursion train in Oct 1954 - Alden Armstrong photo

The next step, I think, will be to start laying out the structure which will be built under the module.  Because I'm getting into differences of grade for the terrain, I'll be tracing the MDF top sheet to the 1" thickness foam.  Then I'll be making some cuts along the roadbed and the foam will also have to be marked for cross structure to support the roadbed and have the foam filling in the areas between the roadbed.

Lots of Woodglue, Gorilla Glue, and ballast weights are in my future to bond everything together!

Jason Hill

Related Articles:

SP Jawbone Branch Index Page - All posts related to my constructing the layout.