O Scale Track Radius

What are you asking here? Are you wanting to run o54 engines on o42 curves?

The prevailing theory in this area has been to always go larger. Depending on the wheel displacement, your 054 engine could hang up on 042 curves.

@Tranquil Hollow RR posted

Depending on the wheel displacement, your 054 engine could hang up on 042 curves.

Thanks for the info! - Kyle

O scale track curves are typically measured in diameter, not radius, so O-42 track has curves that are 42" in diameter and O-54 curves have a 54" diameter. Depending on the type of track, it could be measured from center rail to center rail or otherwise, so you would might have to add about 1.5" per side and end up at 57" in overall diameter - still plenty of room on a 72" wide table.

Given that you're smallest table dimension is 72" wide, you should not have any issue with using O-54 curves and still have room for scenery and structures, depending on how many loops, main lines, sidings, etc. you plan on having.   

One point I would add is go as big as you can given your table size.  Even if you don't have engines that require O54 now, you might want one in the future and trains just look better (even smaller locomotives) on wider curves.

Kyle, it's my opinion that if the gross dimensions of your layout are 12x6 feet, then I would use curves no larger than O42 (42" diameter.)  In fact, I might even use O36 or O31!  Everyone likes the big scale locomotives that require O54 or greater.  But with 12x6, you'll be limited to an oval with short straightaways, really just a wide circle.  And for me personally, that would get boring very quickly.

Traditional-sized trains like RailKing or LionChief, operating on sharper curves, are a better fit for that space.  And a town scene created with Plasticville (which was deliberately sized between O and S gauge), will complement the traditional-sized trains and should fit nicely.  My $.02.