Section 13 - Polson Canyon - Part 1

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

North Canyon

Center Canyon

South Canyon

Layout Design Elements

The Polson Canyon section adjoins the Satsop River section to the north and the Tunnel 1 section 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 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 sections (16 foot long total) and consists of two geographic areas: North, and South Canyon (from geographic north to south):

1. A spur off of the ridgeline with a sparse growth of second growth conivers, 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 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 throught plate girder bridge.
8. One beam bridge.  
9. An embankment with several streams and draws coming down the steep hillside and 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.

Section Construction

See the WWSL section 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 section, 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 section 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 section 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 and 8 inches of river scenic elements. The number of curves must fit three less than 180 degree curves (with easements and tangents) within the 16 foot sections.

1. Abandoned right of way.  There will be two abandoned rights of way. See the following scenery construction pages of the Camp 7 right of way and the Camp 5 right of way.

3. I pulled up a construction diagram of a timber trestle built by the Union Pacific.  That diagram has each bent a distance of 15 feet apart. In HO scale that's a bit less than 2 inches per bent. Five bents and a wood abutment is about 10 inches. 

4. The steel plate girder bridge is an 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.

7/8. There will be two additional bridges in Polson Canyon. Based on the shallow nature of the stream and draw, one bridge will be a 50 foot through plate girder design (See 4.1.9.3 Polson Canyon Bridge N3) and the other bridge will be a 20 foot beam design (See 4.1.9.2 Polson Canyon Bridge N2).

Putting down the roadbed lines 

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 section:

1. I need to layout the curve from the Satsop River section first.
2. I need to layout the curve from the Tunnel 1 section second.
3. I work the curves from each end to the center of the Polson Canyon sections. See the curve template development section below for how to do it.
   
4. 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.
5. 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. 
6. Locate the abutments and piers (to include heights).

Cosmetic reverse curves

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

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

The WWSL sometimes 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 23 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 left half of a turnback curve)

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