My first real layout. The plan is basically a small KISS oval, with the straight section across the back being a double span thru-truss bridge, with a single span thru-girder bridge on one end…followed by a#6 turnout…with the oval curves on either end of this stretch. As is, I’ll be able to remove enough foam under the bridges to give me aproximately 2" under the truss spans for the river bed, and approximately 1" clearance for the road under the girder span, keeping all my track at one constant level.
Now the question…will more clearance under the bridges for appearances be worth the added problem potential from adding a 2% grade on part of the curves approaching the bridges? I’d be using just one section of Woodland Scenics 2% incline starters on each end. I’ll have the incline starters on hand, left over from a graded siding, so there will be no additional cost to be factored in. Due to the small layout size, and budget constraints, the trains I’ll eventually be running will be small, and generally pulled by single 4 axil locomotives.
If your road is a rural “2 ruts in the mud” road, I wouldn’t bother with trying to raise the bridge. If, OTOH, it’s a Federal highway, it should have a minimum of 16 (scale) feet between the top of the pavement and the bottom of the girder. You are a little short in N scale, and a lot short in HO.
Your river isn’t the issue, since the only way you can make it navigable for commercial vessels is to install a lift bridge - or a rack section of railroad to raise it to 50 feet above mean high water. Better to leave it as-is for the fishermen and water skiiers (or kayakers with death wishes, if it’s like my rivers.)
You can always stack more foam layers, and then gouge down to where you want your overpass height. So, if you want the gee-whiz bridge, just have your prairie higher and allow for more depth by adding one or two layers. The rest can just stay flat, or add an elevated/varied relief near the rear for visual effect.
Personally, for a smallish layout, changes in elevation have to be kept minimal unless you are okay with 4% grades. In fact, even with severe compression, on a small layout the grades should be kept below 3%, which effectively precludes them for any overpassing purposes.
It would be a main through route for my little N scale town…probably not a Federal route, but it’s a possibility. I was pretty OK with the 2" to the river bed, but the road clearance was what bugged me. Where I grew up, there were both B&O and Wabash/N&W underpasses that are under 14’, but when I started looking at the Chooch abutments I have, and seeing how much I was going to need to cut off, it got me wondering.
Adding another layer of foam is probably out of the question for me…for both budgetary and accessibility reasons. My prarie level is already going to be at about 36" from the floor, and adding more height to the majority of the layout will make reaching over things from my wheeled chair (mobility issues) more cumbersome than it’s already going to be.
Grade percentage wize, I’d be sticking to the 2% grade of my incline starters, and I’d only be using using one 24" (or less) incline on either end. Using the full 24" would give me a 1/2" more elevation on the bridge. Based on the 16’ figure Chuck mentioned, I could get by with as little as 12" of incline on either side, to make it technically legal.
If you can’t raise the level of the layout could you cut a section out and lower the road bed a couple of inches and have a decending grade for the road that would give you a reasonable clearance under the track, sort of a cookie cutter approach only with a downgrade. A one inch clearance for an underpass will not look right and I doubt you will be happy with it. Cheaper than using Woodland Scenics foam grade to raise the track, and easier than fabricating a grade for the track out of other materials.
That 1" approximation was with lowering the road to a compromise level between the river water level and the surrounding terrain. Without that lowering, I’d only have 1/2". This is one of the few places where cheaper is not an issue…the incline starters that would be used would be leftovers from a graded siding.