A month or so ago, I posted a first description of my modeling efforts to create a creek as a scene divider between Ballard and the town at the end of the branch, Santa Rosalia. This will be Oso Flaco Creek. I described the history behind the creek’s name in that post, and showed the beginning of modeling (you can read it at: http://modelingthesp.blogspot.com/2015/01/modeling-oso-flaco-creek.html ). I also mentioned that one great feature of adding a watercourse to your scenery is that you get to build a railroad bridge. This post is about that bridge.
Southern Pacific, like most railroads, used pile trestles in many locations over the years. Naturally there were standard drawings for these structures, and there were a number of variations on the basic design. Since my trackage over this creek is a branch line, it seemed logical to use the lighter-duty design (Cooper loading E50) shown in SPMW Common Standard drawing CS 1605. The drawing is reproduced in Southern Pacific Lines Common Standard Drawings, Volume 1 (Steam Age Equipment Company, Dunsmuir, CA, 1992), page 65. This drawing is immensely helpful. It shows how wide such a trestle was (16 feet), that its ballasted deck was retained by 4 x 12-inch boards along the sides, that it had 8-inch thick stringers under the ballast, and 12-inch thick caps set atop the piles, and that bents were 10 feet apart. Piles and posts in this drawing are not dimensioned but are drawn at about 12 inches diameter. (You can click on the drawing to enlarge it.)
Other details of interest can be found here too.
The other part of a trestle like this is the bulkhead at each end of the trestle. Here again, SP had a standard design for such bulkheads, shown in the same Volume 1 of Southern Pacific Lines Common Standard Drawings, page 62. It shows that there should a bent of piles against the bulkhead. I know from seeing many such bridges in the prototype that bulkheads varied widely, doubtless to accommodate local conditions, so I did not intend to duplicate the drawing exactly. Indeed, the drawing is likely intended as a starting point for field engineers.
The way I wanted to build the bridge over Oso Flaco Creek is to span the gap with sheet styrene, with track atop it, which later will be ballasted, and add all piles and other details underneath, attached to the sheet. The bridge should of course have straight edges, as seen here in the form of the styrene “deck.”
Before going further, I considered the total thickness of the structure from a side view. The ballast retainer board above the floor is 4 x 12 inches, but its bottom is 8 inches below the bottom of ties. Model track has sub-thickness ties, and these ties will lie directly on the bridge floor, so the net height of the ballast retainer as it will be viewed is at most 4 inches. I chose to use a styrene scale 4 x 4-inch strip atop the floor.
The prototype stringers below the ballast are set solid and are 10 inches deep, and they are supported on 12-inch deep caps atop the piles. I wanted to cut this down so as to leave some height to my piles. I decided my best strategy was to cut the stringer thickness way down, and to use caps of close to the right size.
The layout of piles is given in the uppermost drawing in this post, and I used that drawing to make a paper template for laying out the arrangement. I simply measured the desired dimension on the drawing with an HO-scale ruler to obtain a conversion factor, then reduced the scan accordingly to bring the original drawing to HO, then printed out the reduced version as a template.
I decided to use Evergreen styrene tubing for piles. Evergreen offers 1/8-inch and 3/16-inch tubes. The diameter of the former is about 10 inches in HO scale, the latter about 15 inches. I decided to go with the bigger size. Note there is a spread of these against each bulkhead (see second drawing at top), and one in the center of the creek, since I have a 20-foot span.
I began by making bulkhead faces using Evergreen V-groove styrene sheet with 0.100-inch plank width (about 9 inches in HO). I then glued a pile cap against it to determine the height of each pile, and cut the piles from the styrene tubing, filed a slight flat on the back, and glued them on. Then they were airbrushed the “seasoned brown wood” color from Star Paint (STR-11). Here you see one painted, one unpainted.
I am now ready for these to be glued into place. I will continue with the installation and final appearance of the trestle in a future post.