Grades and bridges on the Siskiyou Line
Better late than never, aye?
I just laid the roadbed and track right through the scene giving little special thought to any grade adjustments due to the location having a bridge. The only consideration I made was to make sure there were no risers in the bridge location.
Then I later came back in and cut out the roadbed and track, although in one case (a curved trestle), I left the rails hanging in mid-air and put the new trestle up under the rails. This location happens to be just shy of the summit of the grade climbing out of Coos Bay, so the bridge actually forms a slight concave curve across it. I also tilted the trestle every-so-slightly to get a banked curve.
Here are some photos of the install process.
NOTE: This part of my layout (the Coos Bay Branch) predates my use of spline. Here I used two layers of 3/8" plywood kerfed every inch about 2/3rd of the way through and then laminated together with glue and curved into the radius I wanted as I went along. I got the method from an MR article in the late 1980s by Rick Rideout.
The Slater Creek trestle location had the roadbed and track built right through the scene with little regard for where the bridge would be. I did plan ahead enough to make sure no risers were in the middle of the bridge location. I also laid the track with shims so it was banked slightly on the curves.
I roughed in the scenery and carefully cut out the roadbed and ties at the bridge location. That's right, I just left the rails hanging in mid-air!
I took the roadbed that was removed and used it to draw a curve template for the trestle on a scrap of drywall. This way I could ensure the new bridge would match the roadbed curve exactly.
As I built the trestle out of styrene, I made sure it matched the roadbed drawing exactly. This meant the hanging rails should drop right down onto the new bridge just as they sat on the roadbed. I used stripwood ties every 5th tie on the trestle so I could later spike down the rails to the bridge.
When I fit the curved trestle up under the rails, it lined up perfectly! After test fitting the trestle into the location, I had built the trestle bents long so I trimmed them to fit the rough scenery contours. Then I mixed a putty-like batch of plaster (with retarder in it to give me about 30 min working time) and trowled it in place where the bridge would go. I fit the trestle under the rails and pressed it down into the thick plaster. Using some shims and c-clamps, I clamped the bridge up against the rails, then used a 1x4 and a paint can to press the trestle down into the plaster. Since I had trimmed the trestle bents to be exactly the right length to set on the rough scenery, I knew the bridge would be well supported. I used a plant mister to spray water down between the bents and wash any unnatural looking globs of plaster off the bents.
Here's the finished styrene trestle with the plaster canyon work complete. To fasten the rails to the bridge, I used spikes I trimmed to about half length and spiked the rails down to stripwood ties (every 5th tie was stripwood). Notice the track on curved trestle is super-elevated slightly. The summit of the grade can be seen just past the trestle to the right.
The finished scene was on the cover of the January 1997 Model Railroader promoting my article about mushroom benchwork.
So the short answer is I first lay the track with little regard for the location of bridges, other than making sure there are no risers in the middle of the bridge location.
Then I make some kind of pattern (rubbing a pencil on a piece of paper over the rails, or using the cut out piece of roadbed as a pattern) and use that to build a bridge that matches the cut out location exactly. Grades don't care if there are bridges under the track or regular roadbed.
