Section 13 - Polson Canyon - Part 1

Its time to layout the Polson Canyon section. The layout design given and druthers of this section is identified in Station 13 Polson Canyon.

Layout Design Elements

The Polson Canyon module adjoins the Satsop River module to the north and the Tunnel 1 module to the south. 

Polson Canyon is a curved river valley with moderately steep hillsides and rock faces, with a sparse growth of second growth conifers, dead trees, stumps and bushes. The right of way parallels a ridge line on the east side (backdrop) and the Satsop River on the west side (fascia) through a series of cosmetic 32 degree (26 inch) curves.

The Polson Canyon section consist of two separate modules (16 foot long total) and consists of two geographic areas: the North Canyon, and the South Canyon (from geographic north to south):

1. A spur off of the ridgeline with a sparse growth of second growth conifers, dead trees, stumps and bushes. An abandoned OPLC right of way at the left (north) end of the module - formerly OPLC Camp 7.
2. A ravine steeply rising to the background with series of rock shelves making a waterfall scene.
3. Two 4 bent timber frame trestles with wood abutments.
4. One 10 panel deck plate girder bridge
5. An abandoned spur - formerly OPLC Camp 5, curving along the ravine.
   
6. A curved river valley with moderately steep hillsides and rock faces. with a sparse growth of second growth conifers, dead trees, stumps and bushes. 
7. One through plate girder bridge located along side a small waterfall.  
8. One timber frame bridge with indications of construction in preparation for a deck plate girder bridge replacement. 
9. An embankment to the river.
10. A heavy growth of shrubs and bushes at the river's edge. 
11. A river's edge along the fascia.

Module Construction

See the WWSL module construction reference page for the construction technique. 

As this section has a river module I have modified the construction technique. Instead of using the two 1x3 girders on the bottom of the module, I need a solid bottom on the module for the riverbed. The 1x3 girders are replaced by a 2 foot by 8 foot piece of plywood 3/4 inches thick. The two ends of the module are cut out to permit attachment of the module electrical connectors and section interface bolts. 

The remainder of the section is built per standard WWSL construction standards.

ROW Design Considerations 

The most critical design considerations on this module is the curvature of the main line track. These curves are 32 degree (26 inch) cosmetic curves. The width of the roadway in the river canyon must be approximately 8 inches in order to have 8 inches of hillside to the rear of the module and 8 inches of river scenic elements to the front. The number of curves must fit three less than 180 degree curves (with easements and tangents) within the 16 foot sections.

1. Camp 7 abandoned right of way.  See 4.2.11.13  Modeling Technique for Removed Roadbed for Camp 7.

3. Two approach curved timber trestles, one on either side of a deck plate girder bridge. There is a Union Pacific Common Standard diagram. That diagram has each bent a distance of 15 feet apart. In HO scale that's a bit less than 2 inches per bent.  See 4.1.9.4 Polson Canyon Bridge N4 for construction details.

4. The steel plate girder bridge is a kitchbatch Atlas bridge with 11 panels. Total length of those two trestle and one steel plate girder bridge will be about 29 inches. See 4.1.9.4 Polson Canyon Bridge N4 for construction details.

5. Camp 5 abandoned right of way. See 4.2.11.12  Modeling Technique for Dismantled Roadbed for Camp 5.

7. A 50 foot through plate girder bridge placed on a curve tangent. See 4.1.9.3 Polson Canyon Bridge N3 for construction details.

8. A  foot through plate girder bridge placed on a curve tangent. (See 4.1.9.2 Polson Canyon Bridge N2 for construction details.

Laying out the Structures

There are no structures on the Polson Canyon module.

Laying out the Right of Way

I am not going to go into great detail about putting down the roadbed lines. See the Right of Way Reference Page  for the details if you haven't had the fun to do it before. Several things to note for this module:

Because this section ties into both the Satsop River Bridge - Section 14 and the Tunnel 1 - Section 12, some layout of those modules will need to be done at the same time: 1) the main line locator at Satsop River (12 inches from the fascia) and the northern Polson Canyon curve (16 inches from the fascia) and 2) the southern Polson Canyon tangent and the northern Tunnel 1 curve (6 inches from the fascia). The Tunnel 1 module is a turnback curve and extends 6 inches beyond the Polson Canyon section fascia.

Cosmetic reverse curves

North Canyon

Center Canyon

South Canyon

Looking at the photos above, a model railroader would probably call the track layout an S-curve. Calling this section of roadway an S-curve isn't prototypically accurate. The railroads just see this as a series of curves and engineers them appropriately. 

In model railroad layout design there are actually five functional curve types and one cosmetic curve type. This photo would be considered a series of cosmetic reverse curves. See 0.4.5 Curves for additional details.

The WWSL usually uses easements on its main line right of way. I am modeling a Cosmetic Reverse Curve for the NMRA Master Model Railroad Certification Program. You may choose to eliminate the easement in each reverse curve to simplify right of way construction. In this section there are six elements in the roadway construction:

1. easement  - simple curve  - easement  - Bridge N4 (tangent)  
2. easement  - simple curve - easement - North tangent   -
3. easement  - simple curve  - easement - Bridge N3  (tangent)
4. easement  - simple curve - easement - Rock wall   (tangent)
5. easement  - simple curve - easement - Bridge N2   (tangent)
6. easement  - simple curve x2  - Tunnel 1 (the northern half of a turnback curve)

I tried a number of techniques in an effort to lay this all out. What finally worked was using Atlas Code 100 flex track (it's incredibly flexible) and both Ribbon Rail and Fast Track Sweep Sticks straight and curved track gauges to build an 'easement  - simple curve  - easement  -  tangent' tool. With that tool it was just a matter of laying out the curves and adjusting as necessary.

See 4.2.4.4 Laying out a Cosmetic Reverse Curve for additional information.

1. Layout the cosmetic curve boundaries. I placed those lines at 8 inches and 16 inches from the fascia.
2. Layout the curve from the Satsop River module to Bridge N4.
3. Layout the turnback curve from the Tunnel 1 module.
4. I work the curves from each end to the center of the Polson Canyon sections. The center canyon tangent is at 14 inches from the fascia.
5. I can adjust the locations of the curves with track tangents in the center of the two inner (concave) curves  and/or the center of the two outer (convex) curves.
6. The Polson Canyon bridges are centered on the curve tangents. Bridges N2 and N3 are on tangents with no additional calculations needed other than abutment placing. Bridge N4's plate girder bridge length must be determined in advanced so as to properly place the point of curvature for the northern-most curve. 
7. Locate the abutments and piers (to include heights).
   

Roadway.

1st (Northern) Division is

• The WWSL 1st Division is designated Class B main line. See See 4.2.11.2 Modeling Technique for Medium Profile Roadway for details.

Layout Drainage.  The embankment along the center canyon area has a number of possibilities for drainage. In adddition to the roadway ditches there will be a series of pipe and box culverts. See 4.1.8 Ditches, Drains and Culverts for general information and 4.1.8.8 Pipe Culverts and 4.1.8.9 Box Culverts for specific details.

Track Details.

In addition to identifying cantenary pole locations, I am considering a slide fence in the center canyon area and one or more cattle guards. 

In the next blog (Section 13 - Polson Canyon - Part 2) I will discuss the Landform Design Considerations.