Have a look at John Allan’s ...

... very first Gorre and Daphetid track plan.  It’s proven, it’s timeless, and here are track plans and at least the beginning of a construction thread from MRH:  G&D, and here, in current context the same layout as the beginnings of a complete replica of The Master’s masterpiece  You could,have the beginnings ...

You did say “ish” so I wasn’t sure if you have the latitude to add 6” each way but done with care,I think this would be an outstanding little pike. I’d be inclined to just go flat top and use Woodland Scenics foam pieces for achieving track elevations.

Or maybe just build it in N!  You could get a lot more railroad in 2’ x 4’ with N Scale.

HOn2 1/2 is another option and would be happy with 12” to 15” curves.  HO on N Scale mechanisms.

Honestly, I'd find a way to

Honestly, I'd find a way to push out to 38"-39" and go with 18" curves. I'd probably also make it a City/Industrial scene. Not sure if that fits with your preferences.  

The radius is set..

Hi JM, 

If you are using sectional track then your radius is set by the brand of sectional track you are using. Remember these specified radius measurements are to the track centre. 

Kato HO Unitrack lists 430mm (16 ⁷/₈") radius as the only one that can be turned around in less than 3' diameter. And that might be a tight fit - remember most HO rolling stock is wider than than your rails and ties. Stuff overhangs more as it enters and leaves curves than it does on straight track.  

Peco list their #1 radius at 371mm or 14.6". This will fit OK. Their #2 radius at 438mm or 17.2" will be too tight a fit to work on a 3' maximum diameter. 

Atlas #1 radius specified as 15" radius. 

Either Atlas #1 radius or Peco #1 radius options are probably your safest bet at fitting your dimensions without rubbing on your lexan safety barriers along the long edge of your proposed layout. 

Remember - the OP has stated his size is fixed. 

What's important to you?

Dear J,

Gotta ask, what's important to you?

- Do you primarily want to see your trains running thru scenery?
(are you particularly interested in the visual appearance and "trains in scenic context" of model railroading?)

OR

- Do you primarily want to run/operate trains, (it's all about train-movement, purposeful or otherwise),
and the visual is secondary?

The answer will help guide you, as:

- for Visual and "Trains in Context" performance, that extra room between the track and the edge can be key,
(IE requiring a tighter minimum radii curve)
and you may find that slewing/rotating the "oval of track" mainline so the straights aren't parallel to the baseboard edges, even if only by a few degrees, can be a serious game-changer in scene and scenery design and implementation.

- for Operating, esp if despite the modest layout footprint, you're hell-bent of running "real trains",
(IE it's Geeps and SDs and 50'ers, this "artificial switcher + 40' cars" scenario feels too contrived),
then the larger radii really is going to be the better investment. What you loose in the visual you will gain in mechanical "elbow room", even if the actual linear-track-length is still a hamstring...

Personally, for the overall size of the layout,
(which IMHO is entirely capable of being both a great "starter layout",
and capable of supporting long-term design/build/ops interest if treated with due respect),

I would strongly reccomend keeping the "equipment context" down in the "Switcher + 40'ers" headspace,
(shorter cars = more "linear capacity")

and focus on shortline type ops a la Mike Kieran's "Port Able" shortline, or the Cripple Creek Central "HO Railroad you can Build"...

Happy Modelling,
Aim to Improve,
Prof Klyzlr

PS FWIW 1 metre x 1.2 metre fold-up layout,
inc powered B'mann EZ-Track turnouts and 18" radii curves,
built for a neighbour's child and Granddad to share running of Granddad's old (now repowered by yours-truly)
Tyco "Chatanooga Choo Choo"...

...which by co-incidence, even when "half folded", actually works surprisingly well as a car-spot based "tuning fork" or "single turnout" switching micro-layout...
(pass the SW7, 40' cars, and carspot switchlist...)

I think that some of the rolling stock will hit the lexan &

derail with the track going right to the edge of the layout.  Even if your 40 foot & 50 foot cars don't have enough overhang to hit the lexan, your locomotives will probably hit it.  You may want to test with the 15 inch radius.  The op may want to reconsider the "fixed size" of the benchwork.  Adding as little as 2 inches will probably provide the necessary clearance with the 17 inch radius.  I think 4 axle "Sw" type locomotives would clear the lexan, but I think 4 axle gp types will hit the lexan in the curves. 

The other radius I could use would be 15".

If Kato makes 15 inch radius I'd go with that instead of running the track right out to the edge of the layout. I'd first think about using 17 inch radius and adding a bit of width though.  I had a layout with 15 inch radius Atlas snap track long ago and it worked fine with small engines and rolling stock so it's possible to get by with that radius curves....DaveB

If you’re set on 3’x6’

why use sectional track? Why not atlas code 83 and draw your own radius at say17”, that gives you an inch on each side. Use no roadbed, set it up as rural branch, cinders and dirt as ballast right over ties, small switcher like a 44 tonner, 40’ cars. I’m not sure what you want to model.

My Experience

I have used some Atlas 15" R curves in industrial areas with no issues with SW type locos and 40' and 50' cars. Do a test run with the Lexan before you attach it to the edge of your layout.

Jack 

You should really do whatever

You should really do whatever possible to make the layout slightly "wider".
I'd use the Kato 18.75" radius track (I think that's what it's close to) as "my minimum".

Anything less will severely limit the locos and cars you can run on it.

Being limited by your turn radius is really "freedom"

Seriously, enjoy all aspects of the layout.  A 3ft x 6ft HO plan can be enjoyed, if you are willing to live with the limitations.  You can have switching operations, loop running, and scenery by using a 15" radius.  No, you can't have large steam, passenger cars other than MDC Sierra Ry 34ft passenger cars, or freight cars longer than 50ft.  No, you can't have 20 or 30 car trains.

But you can have fun with a diesel or steam switcher or even a 4-4-0 or 2-6-0 and a dozen cars on the layout.  You can have some scenery and switching ops, as well as loop running.  You can build small structure kits, and apply your hand at designing and building scenery.  You can even try your hand at building NOS steam loco kits from Mantua or Roundhouse should your desires lean in that direction.

Or go back in time to early 20th Century or late 19th Century, when locomotives and cars and industries were smaller (no truck service available).  Build some wood car kits - Labelle still makes them.  Or buy some "junk" Civil War train set cars and locos and fix them up to current detail and running standards.

And you won't have sunk a fortune in the layout should the time come when you have more space.  In the meantime, you won't be tempted to buy those large locomotives that won't run on your layout.  Any of the above beats not having any layout at all.

Fred W

....modeling foggy coastal Oregon, where it's always 1900....