Measure you tallest rolling stock.

How far does the track need to traverse under neath the ductwork with the tight clearance?  If the ductwork is only 6 or 8 inches in diameter, and the rolling stock will clear, you should be ok.  If something derails directly under the ductwork, you can fish it out and re-rail where you have more clearance.

  There must be a way of

There must be a way of gaining just another quarter inch. Maybe no roadbed in that area or even a slight downgrade followed by a slight upgrade? This quarter inch dip/rise can be stretched out over a foot or more. You will be much happier with just a little more clearance rather than losing the detail from the top of your locomotives and rolling stock.

Pete

2.75" should suffice

We set our minimum clearance of 3 inches exactly from below the track base and that serves well for 70s era Plate F equipment like autoracks and hi cube boxcars.  We would be in trouble if we ran doublestacks, but they hadn't been invented yet back then.  For the 50s you would only have to deal with some Plate C cars, which are quite a bit shorter than an autorack, so I think you can get by with 2.75 inches.  Plate C cars work out to about 2.14 inches high in HO.

Lower benchwork?

And why you just don't build the whole benchwork 1/2" lower?.

Reason for not lowering benchwork...

Because of the nature of the mounting bracket for the shelf bracket's I would have to lower everying by 1", which causes problems with having the storage carts clear underneath the bottom edge of the benchwork.  It already is "very tight" (within 1/2") clearance with the carts.  I'm trying to stuff 100 pounds of "stuff" in a 10 pound container, but if I am going to have anything close to what I would consider reasonable, this is how it needs to be, or else, I would have to say why bother doing any of it.

Ken L.

How thick is your bench work, Ken?

I ask because if your bench frame work is 1x3 or 1x4 dimensional lumber topped with 1/2 or thicker plywood, you could reduce the thickness by using a 1x2 frame in that area and luan for a table top.  Stiffen the luan by using 1x2 cross members on 12 inch centers.  Then if you are using foam below the roadbed, trim it down to get your extra 1/4 inch of clearance.

Benchwork Thickness

is between 2.5" - 3".  For the Staging Deck it is 2.75".  The 2x2 Metal studs/tracks are 1.75" and for the staging deck i'm using 3/4"" MDF (from a previous layout that I want to "recycle") and for the layout i'm using 1" foam board.  I do not want to deviate from this form of construction.

Ken L.

Well.....

..... then the last suggestion would be to bang up the duct a little bit, or stick with low profile rolling stock. Sometimes something has to give up. Trains are fragile... and expensive. Or else.

Ductwork

I don't know what the duct work looks like but maybe you could change out a short section of ducting for something with a lower profile. Keep the same square area of opening. A tin man can bend up anything you want.

Ductwork

A picture of your particular situation would help.

Use a bridge as a clearance test.

Put the track in, and then measure exacty how much clearance there is.  Then when you build the tracks on the visible layout approaching the tight spot, model low road bridges over the tracks before the trains get to the ducts, and make sure the clearance under the road bridges is slightly lower than under the duct.  That way you will have peace of mind that if your train gets under the bridge, it won't get stuck under the duct.  There were hundreds of examples of low road bridges which restricted the height of equipment which could run even on mainlines.  On the Sudbury Division we are building an old steel bridge over the east end of Sudbury yard which will barely clear an autorack car, because that's what was actually there in the 70s.  The bridge had to be replaced with a high level concrete bridge before doublestack trains could run on the line.

I got your fudge factor right here...

Ken - I have one spot on my layout where the clearance from the top of the cork roadbed to the bottom of some backdrop framing is exactly 2.75"; just re-measured it a minute ago to be sure.  I could reach in and trim the framing, but everything I run clears easily, including enclosed tri-level auto racks.  I've left the tight clearance as-is since nothing I operate has a problem with it.  You should be fine.

Thanks Rob!

That is what I was looking for, someone who has such a tight clearance and hasn't had any issue.  I can now proceed  with mounting the wall brackets and then the actual benchwork!  

Ken L.

Clearance

2.75" is 19 feet.  Assuming that this is the clearance from the top of rail and not from the benchwork or roadbed, that will clear plate B or C cars which have to be less than 15' 1" tall. 

So assuming you will not be running autoracks, high cubes, high loads, double stacks or engines with stack extensions, you will clear.

Re: Dave

Note that the location I described at 2.75" above roadbed height (meaning a bit less from the top of the rail) still clears enclosed autoracks and high cube cars.  I have a few of the former and many of the latter.  I haven't tried double stacks or any odd excess height loads.  If I change eras I might have to do some work in there.