How to build an arched two tier trestle? Steam & Jig?

How about cabinet grade plywood or use 3/4" MDF. Just cut your entire shape out of sheet material.

I may have misinterpreted your goal. You want to curve the bridge to the track diameter. In that case it would be much more difficult. Bending may have to be done cutting kerfs in one side to allow bending.

If you have the room, make short straight sections with angled ends so it comes out like an octagon pattern.

I could likely help you.

I would make the bridges for the straight sections rather than build curved ones. I'd also construct multiple bridges given the track configuration on the various levels.

I like the "frame and skins" method for building a bridge. Use 3/4" plywood for the deck and quality lumber for the supports. Make arches from 1/4" plywood. Trim up and paint it like aged concrete and you're good to go. You can make any number of arches you want. In the example below, I chose a single parabolic arch.

I'm currently working on the design for a replica of the Pacific Electric Torrance Bridge.

The basic core of this bridge is composite decking material, which is very heavy. The longer bridge at the other end of the viaduct was done using select pine, but if you can't get good straight lumber, plywood is the best bet. The sides here were done using 1/4" MDF, but the subsequent bridge was done using 1/4" plywood sides. 

Seal it up with sandable primer from your local home improvement center. Kills any trace of wood grain.

The surface was spray painted with stone textured paint, then that was sprayed with a coat of "Straw Flower" to give it a yellowed look.

The railings were cut from the plastic screen used to keep leaves out of rain gutters. One of our members had some lying around and it worked out well.

Here's what the skeleton and skins look like. The deck was cut short so that the adjacent road bed would rest on the core of the piers. The Art Deco styled columns were made from moulding (doubled up) with the ends cut and rounded off.

Here's the rest of the viaduct with the second bridge. Though this bridge is longer, it weighs less than the first one because it was done using lighter materials. 

Thanks for the replies everyone. I may be getting back to you Tim.

Matt that’s a great looking bridge and the process is what I will consider if my current experiments don’t work out. In the end I may have to give up on bridging the curved sections.

I spent most of the afternoon yesterday digging my vertical steamer out of the garage attic. It’s been about 15 years since I’ve used it and I forgot how big the darn thing is. It’s shaped like a rocket and can accommodate 65” lengths of material.

 Since I went through all the trouble to pull the thing out, I decided to run a few experiments. I ripped a scrap pine 1’ X 4’ down to 3/8” x 3/8”  and ¼” x ¼” .

Set up the steamer and it’s been running since about 7:00 last night. I’m going to try using a empty gallon paint can as the jig to get the curve I want. As soon as things warm up a bit here I’ll try setting the material in the jig.

The reason  I want to use more of a skeletal frame trestle is to keep the weight down, the display has to be moved quite often and also keep the view from front to back as unobstructed as possible. This is only a 5’ X 8’ display which is generally controlled while lying in bed. When it comes to train operations I’m one lazy SOB.

Thank you and best regards everyone. I’ll continue to post my progress or failures which ever the case.

You can use Bender Plywood . It is made so it bends I have used it in a lot of projects. There many places to get this stuff, this was just the first one I found. Just search Bender Plywood. 

The  back of this was about 40"dim . It will bend much tighter then that. Comes in different thickness, the thinner the tighter it will bend. 

Great work on that bridge!!! Thanks for sharing!

Is there a reason you will use plywood over MDF in the future??? I just bought some MDF for a project and really like it over ply......is there a down side to MDF?? THX

Major failure in my experiments. But I did discover soft woods such as pine are not good for bending. Hardwood is easier to bend but you want it to be air dried and not kilned dried which I believe pretty much eliminates big box stores as a source for material. Air dried oak may be the best wood for creating tight radiuses. Vertical steamers aren't preferred steam chambers for this application, horizontal steam boxes are easier to work with because of the short steam periods and working times involved. Surprisingly I discovered wood can be over cooked and brittle if left in the steamer to long. Anyway I have a nice pile of kindling available. 

Weight and structural considerations. MDF doesn't take carpenter's glue as well as plywood -- the parts can be peeled off because MDF is made from presses paper and the glued layer just peels off. It's also not as structurally strong in the 1/4" panels. I built a warehouse shell (background full building 48"w x 20"h x 9"d) as a view block with it and had some problems with breakage on a couple of columns, plus the thing got pretty heavy.

Call Bridge boss or the NJ Hirailers, both have solutions for a curved bridge.