Are 4" track centers OK

You should have absolutely no problem with 4" center to center on those large curves.  I run everything, including full length 80' passenger cars, around 72", 68" and 64" radius curves on my layout with no interference.  My tracks are on 4" centers.

Joe Foehrkolb

I run 80 ft passenger cars on 52, 56, 60 inch radius curves and have no problems.   Mine are weaver and GGD, so they themselves handle the curvers quite well.    

I have seen high brass passenger cars that balk at such curves, but I think 72 will still work fine.

You'll be fine. Even a Big Boy doesn't swing out much on 72" radius. I made an inquiry about a year ago when I was considering building some two-track curve modules at 72"/74" radius and got these photos (this side of the house has been incredibly enlightening). From the look of things, a Big Boy (worst case scenario) on 72" clears a full-sized passenger car by about 3/8".

I remember that thread from a year ago.  I saved your pictures for future reference during my layout build.

Those pictures convinced me to go to the widest radius I could fit.  I have all large UP steam and full length passenger cars, so it only made sense.  Even at 72" radius, the BB had more overhang than I expected!

Thanks again for the pictures. 

For a NMRA comprehensive overview:

Matt,

Excellent!  You showed the worst case scenario (an articulated with large overhang on the inside track and an 80' passenger car on the outside track) and 72" R works just fine.  Tom provided the NMRA specs and for 72"R, the NMRA suggests 4 5/16" O.C.

Ed

The surprises are in the MFG variables of boiler length, actual mechanism geometry & passenger car bolster pivot point.  (The first MTH BB boiler was 3 scale feet over length).

Just in case you are running larger non-articulateds I'd test them as well. Those would be 4-8-4's , 2-10-4's and that monster rigid frame UP with the 12 drive wheels that escape's me at this time.

Ahhh.....the "subtle" difference between radius and diameter.   

Large steam looks better and better the wider the curves are.  Unfortunately, limited space ultimately rears its ugly head.

Thanks for all the answers and photos. I can now finish up the layout drawing and then change it another 90 times.

Ken

Hello car8534,

You will be fine, I have Atlas O track, tried to use the NMRA standard of 4-5/16",

I ended up using 4-1/4", one spot on two tangent curves coming together is at

3-13/16" with no problems of 80 passenger cars and cab-forwards passing flawlessly.

ncng

ncng,

I know what you mean.  That is good info, thanks.

Ron