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

Design Elements

Polson Canyon is a river valley. The right of way parallels a ridge line on the east side and the Satsop River on the west side. 

This Section consist of two separate sections (16 foot long total) and consists of eight layout design elements (from geographic north to south):

1. The northern background is a clear blue sky suggesting a sunny day with a mid distant tree line of full forest of Douglas fir. The eastern background is a clear blue sky with a sparse conifer ridgeline.
2. An abandoned curved spur at the left (north) end of the module - formerly OPLC Camp 7.
3. A spur off of the ridgeline with a sparse growth of second growth conivers, dead trees, stumps and bushes.
4. A ravine steeply rising to the background with series of rock shelves making a waterfall scene.
   
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. A series of cosmetic 32 degree (26 inch) curves.
8. An embankment with several streams and draws coming down the hillside to the river.
   
9. The Middle Fork of the Satsop River running roughly parallel to the main line in the forefront of the section.

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.

(pic)

Scenery Design Considerations

1. The backdrop available for the Polson Canyon section is 22 inches in height. It will be a simple backdrop - a clear blue sky suggesting a sunny day.

2. The hillsides should be at least 3x the height of the train. Using 2 inch styrofoam insulation, this would make the hillsides at least 7 inches high with a maximum of 11 inches at the rear of the section. Rock faces would have to be at least 6 inches high to provide a reason for the curvature of the main line thru the river valley.

3. The Middle Fork of the Satsop River runs parallel to the main line in the front of the section. The river will be approximately centered on the module and will disappear to the left and right front. The river bank will be a slow rise to the right of way. The river bottom is composed of a large number of various sized stones and gravel. A shallow pour of properly tinted resin and the creation of rough water will round out the overall look and feel of the river.

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.

2. 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. The northern-most bridge over the ravine will be a wood abutment.with a 4 bent timber frame trestle, one 10 panel plate girder bridge, a second 4 bent timber frame trestle with wood abutments The ravine will rise toward the backdrop and have a wide stream with waterfall. 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. 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.

4. 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 appropriately. In layout design there are actually five functional curve types and one cosmetic curve type. See 0.4.5 Curves for additional details. This photo actually is called a series of cosmetic reverse curves. There are 23 elements in its 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)

The WWSL uses easements on its main line right of way. See 4.2.4.4 Laying out a Cosmetic Reverse Curve for additional information. Eliminating the easement in each reverse curve simplifies the right of way construction.