You could look into a small

You could look into a small hanging Nu-Air unit.

Shop Vac sitting in the empty

Shop Vac sitting in the empty space in the helix with the hose running through a wall adapter?  

jeff

vac in the helix

That's an interesting idea. Hadn't thought of that.

Wonder how much it would pick up. I guess I could 'disturb' the dust in the main room such that it might get airborne enough to get picked up by that vac hose?

Catch it at the source

Your best bet is to stop the dust from accumulating rather than waiting to collect it after it gets around.  Don't do things that create dust storms in the layout room.  If you have to use a power tool on the layout that's going to create dust, figure out how to have a shop vac held as close as possible to what's making the dust.

One of those hanging dust extractors may help too, but it's no substitute for capturing the dust at the source.

Box fan w/filter

Go out and buy a box fan about 20"x20". Buy a 20"x20" furnace filter. 

Tape the filter to the "air comes in" side of the box fan. 

Stick it in the railroad room. 

Turn it on. 

Change out the filter when it turns brownish. 

It's not a perfect solution, but it is cheap and you can just leave it running 24x7. 

Use a Stanley Surform Tool

to clean up the cork roadbed. Does a good job and does not throw dust into the air. Easy vacuum cleanup after. I did all of my cork ( and a little homosote) with the tool. Also good for cutting tapers from one level to another.

Espee over Donner

ps- no relation to the Stanley tool company that I am aware of

That's an interesting idea.

Laminar air flow from ceiling to floor, with a little positive pressure inside the room that will force airborne particles out of the space. That is how a clean room works.   

Just put your vacuum outlet house into the the room with the suction hose on the outside,  room will pressurize forcing dirt and dust out an opening in the floor or vent low down in the space.  Blow everything off and watch all the bust exit the small vent.  Make sure everything is closed off no open doors or windows etc.  as this will be an esca[e point for the air and you will loose that positive pressure.

Using Positive Pressure

I used to do that kind of thing; mostly hospitals and labs. The problem with small, structurally-tight spaces, like most layout spaces, is that controlling the pressure differential is more difficult than in larger spaces. The parameters are much tighter. Conversely, small spaces with a relatively high leakage rate are easier to control. There's some engineering involved; I'd go with Jeff Schultz's idea. 

Obviously 'high wind' would

Obviously 'high wind' would be an issue but I am using a battery powered leaf blower to frequently clean out our garage. It gets dust airborne and if the outside wind is right it will really do the job with very minimal effort.

I have a variable speed leaf

I have a variable speed leaf blower that I thought might come in handy?

I also have 'communication holes' in the back wall of the room to the helix structure, So if I blocked out all but one and used the shop vac in the helix idea, that would be simple enough to run a test.

...and those 'communication holes' exist on all 3 layers of my layout

!!!

Are you trying to create a scale tornado???

mitigating dust

Don't use a leaf blower unless you want the dirt to end up everywhere that makes it even a bigger problem to remove later.

 Also, depending on the stage of the layout there is a huge difference in how you approach the dust/dirt problem.

My little layout is populated with a lot of complex structures and scenery making cleaning difficult. I would not use a shop vac inside the space at this point. In a pinch a long hose and the shop vac exhausting outside the layout room is acceptable. During benchwork construction shop vac inside space is OK.........leaf blower.....nope.

Besides careful cleaning by hand I've implemented two dust fighting measures. 

Cork Dust

Yesterday I was sanding a taper into some more cork roadbed,...BUT this time I moved the piece outdoors in my carport.  Boy am I glad I did !

Look at this fine dust mess.
 

This is the stuff that is still 'drifting' around my train room.

I saw this ad today that started out touting its capability to decrease dust in a home,...BUT when I read the reviews its seems as though they are ALL touting its odor control,...not its dust control ??
https://naturefreshpurifier.com/

Ryobi hand held problem

I don't know whether to call this stupidity or just oversight on my part 

Yesterday I was working on another section of my stone arch bridge and need to some more of that annoying cork dust out of the way. Why does it seem to hang around and just get 'redistributed' ? 

Well, I discovered one of the problems. My Ryobi hand held vacuum seemed to be doing such a good job of picking up that cork dust. i discovered a lot of the very small particles were exiting out the relief vents !! You couldn't even see it streaming out because it was such small particle size, but when I looked closely at areas i had just vacuumed and had moved on to an ajacent area, I looked back and found 'new' dust in the area i had just cleaned.

I inspected some of the gaskets on the vacuum and the tight fit need to prevent this, and found that one has to be very firm when cleaning out this unit and reassembling it. ....one lesson learned. In many cases I was just redisributing my dust.