This post will show how to make the ladder for the curves. I took the pix in this post during one morning - 7/17/20. The time it took from the first pic to the last was about 4 hours. The only step needed after this is to ballast the track. I ran a train to end of track after the last pic was taken to see if there were any places that caused derailments. An Aristo SD45 was used because its trucks are plainly the most picky when it comes to bad track. No problems.
So here we go. First, you need a straight section 2x4 installed as explained previously in this thread:
Note how the terrain has been dug out about 2" with the width of 8" for the entire length of the planned roadway (the ditch on the left is the actual watercourse, the roadway is on the right):
Next, place a length of the PVC trim onto the roadway. Note: you want to use a length of PVC that will completely go all the way from one 2x4 straight section to the next. If you have to join multiple PVC trim pieces to get the required length, the same PVC glue used to join PVC pipes works perfectly and permanently. I always prime the joint first with "Purple Primer". Note that this PVC trim piece is actually 2, for a 16' length. You can see the joint to the right of the bridge pier.
I always put down the outer PVC curve piece first, just a habit. It doesn't matter. Now put a screw thru the PVC trim piece and into the 2x4 straight. Just use one screw now, near the end of 2x4 so you adjust the angle of the PVC piece. The top of the PVC trim piece is even with the top of the 2x4 and overlaps the 2x4 by a couple of inches. Later, after the entire PVC ladder is adjusted to the angle (grade) you want coming off the 2x4, put another screw behind the first:
With the outer PVC curve in place, attach short pieces of 2x4's to the inside of the outer PVC. It was suggested to cut the 2x4's width down so as to support the track better. I did this and it worked out well. (Thanks Pete Thornton!). Use 2 screws on the outer PVC piece and 1 on the inside to keep these short pieces from wobbling when you attach the track to them by nailing or screwing. I cut them down to about 2.5" in width. If you want superelevation, attach them so their top flat area is a little distance below the outer PVC trim piece's top surface. You can attach the inner PVC trim even with the top surface so there will be a natural tilt to the inside of the track.
I put them in about 1.5 - 2' apart:
Now make sure your curve is to your liking and attach the other end to the next 2x4 straight. I'm attaching the other end here to a bridge abutment, but the outcome is the same - you are attaching to a fixed structure:
It is now a complete curve, it just needs the inner PVC trim piece. Don't let the outer PVC trim piece move while you attach the inner PVC trim piece. When done, the curve will stay in place:
At this point, you can fill in between the ladder with whatever fill material you use. I just used dirt and small rocks smoothed to the geometry of curve:
Now just lay down your track. I use flex track because it gives a natural spiral transition and is easy to conform to the ladder: