Mianne

Mianne and Sievers offer products for building above the first level, don't think they would do that if they were afraid their benched would fall. They won't fall, and you will find several Mianne layouts here that are more than one level.

Mianne makes use of manufactured I-beams using MDF for the web and I believe the top and bottom rails are poplar, or some other softer hardwood. This I-beam type design is used a lot in home construction for floor joist. Light weight and strong when it is all locked together.

Another site to consider is model railroad benchwork dot com. That product is similar to Sievers except they use plywood instead of solid, select pine. If you check out this site you will notice he offers curved benchwork, if that makes any difference to your plans. The others approximate curves. I am building my layout in this fashion, ripping plywood down to 3 1/2" wide strips, with pocket screw joints.

Any of these products will work well for multi-level layouts. I am glad you don't seem to be in the camp to  just throw more 2x4's at the project.

I have used Mianne, I think you might be interested in this post 3 Level Mianne

I have been greatly impressed by my Sievers.  Beautiful wood look and solid.  Went together so quickly too.

One difference I see between bench work systems is the spacing between joists/crossmembers. Those who use L-girder tend to use 16” center-to-center spacing, though that spacing can be easily customized (16”-20” is recommended). They then tend to use risers to raise/lower sections of track using cookie-cutter style sub-roadbed rather than the flat tabletops like those in the link Bill posted, basically 3 flat tables stacked on top of each other. They use tabletops in yards, etc.

Mianne uses 24” spacing while Sievers uses 12”, though it does have a 16” section. Mianne moduless are easily extended in increments of 6” without duplicating end pieces. Sievers modules are complete and come in 16”, 24”, 30”, 36” and 48” configurations. When connecting modules, you end up with 2 end pieces attached the each other. Even small corner filler modules are complete rather than just adding a simple angled piece. Mianne’s connection system does leave a portion of the top of the leg exposed if the tabletop is laid between the edges of the I-beam. You can see that in some of the photos of them 3-level layout.

Like L-girder, Sievers makes risers of varying heights while Mianne makes a filler piece that fits in the I-beam so custom risers can be attached. However, I don’t think I’ve ever seen a Mianne layout that doesn’t use tabletop style decking. Risers are still used on tabletops, they’re just custom length supports set on the table rather than attached to a crossmember.

Sievers recommends legs every 36”, Mianne every 48”, L-girder 72” or more. Of course, that varies with the design. The biggest advantages to L-girder are that it arguably uses less material and is easier to customize. The biggest disadvantage is it doesn’t come in kits, so there’s a lot of cutting involved and that takes time, tools and expertise.

I have the Mianne benchwork topped with 1/2" Baltic Birch and 1/2" Homasote.  Most of the legs are spaced at four foot increments, and I have no issues with 170 pounds walking around on the top.

I'm generally happy with the Mianne benchwork, obviously with any product like this you will have some little complaints, but you'll have no problem building a quality layout with the Mianne as the base.

Here's a link where you can see what I built:  New Train Room Finally Getting A Layout!

Here's the Mianne structure assembled before the top went on.

This is what the "final" structure looks like before any scenery.

Mianne suggests that I put plywood on all of my first level. What if I have a couple switches, some  track and a couple houses in an area  on the second level. Wouldn’t it be easier to have the first level open under that area with no plywood so that I could reach up from the floor up thru the first level to wire track, switches etc  on the second level?

The plywood gives it diagonal support, but you can cut holes in it for access to the upper level.  I have several holes in mine for access.

I believe all of these bench work products depend on the plywood to add the last bit of rigidity. Another option is adding gussets in the corners of sections you want to have open.

If you have the plywood all around the area, it should provide sufficient diagonal support.  If you have a lot of open area, then I suspect something like gussets might be needed.

My access hatch was cut between the support beams of the Mianne benchwork, and it didn't affect the rigidity at all.