I use......

I Use 2.5" on Curves

I think they are about 30.5 and 33 in my helix. Never been a problem. For smaller radii you might need to go even wider.  It is easy to set up a couple of lengths of flex track on a curve and test what clearances you need for your self. Even though I model in the 50's, I established all my clearances for longer (autoracks) and double stacks when I built my railroad. That way I never had to worry whether something would clear.

Espee over Donner

Outer (outside) Clearance?

So in both cases here its suggested that 2.5 inches between the centerlines of the 2 tracks should work out.

That insinuates that it should only require 1.25" between the centerline of either track to a 'imaginary barrier' between the 2 tracks?

And that further insinuates that a clearance of only 1.25" to any obstacle outside the centerline of the outer track would be needed??  

I keep remember seeing some images such as this

Those autoracks are on the 26" curve, and it appears barely enough for them, although that flatbed has the same footprint as autoracks and it seems it could make the 24".

This is Where Mocking Up has an Advantage

Place your track. Place the desired cars on the track. Using a vertical straightedge mark DOWN ON THE BASE the farthest on the outside and nearest on the inside that the clearance of the car or loco calls for. Then add a liitle for safety in construction. Don't forget that the end of a car or locomotive may protrude more than the middle. Check both forward and backward especially with locomotives. This isn't about absolute numbers, it's about what works.

Espee over Donner

I'm thinking, Wow those ae

I'm thinking, Wow those ae some tight curves. 

Min curve on my layout is 19"

NMRA recommended practices for track spacing on curves

The NMRA updated its recommended practices for track spacing on curves last year.  RP-7.2 provides recommended spacing by scale, era (size of typical rolling stock), and curve radius.  Bigger equipment and tighter curves require broader spacing between tracks.

You mention running autoracks, which implies the "modern" equipment standard.  The recommended minimum spacing for a 31-inch curve is 2-9/16".  That can narrow to just 1-15/16" for tangent (straight) track - though many people round that up to 2".

These NMRA standards are on the web at https://www.nmra.org/sites/default/files/standards/sandrp/pdf/rp-7.2_curved_track_centers_july_2017.pdf

-- Steve

I never have read the NMRA

I never have read the NMRA figures, but they are very close to what John Armstrong has in his Track Planning for Realistic Operation.  Still a recommended track planning book.

Experiment

I'm going to grab some flex track and some long cars and locos out of my storage trailer today and run some experiments. I'll take some photos.

Real railroads often place

Real railroads often place parallel tracks as close together as possible. There is an article on this topic with a photo of one the Western Maryland’s yards where the hoppers on a curve have about 6” of clearance. 

This is one of the things that can vary widely in real life. The former PRR mainline has clearance for two tracks in between the two running tracks since Conrail/ Amtrak removed the extra tracks and they are trying to make it a 125 mph high speed line.  Most modelers place tracks wide out of an abundance of caution and how hard it is to fix the issue of too tight of clearance after you have sceniced everything. I currently tend to place industrial/yard tracks as close together as I can get away with. 

I found this site helpful

This is what I followed and has seemed to work:

  http://shelflayouts.com/model-railroad-layout-design/design-best-practices  

-Matt 

Long Passenger Cars & the GG1 loco

I did some testing out on my 'outdoor test bench' . As I have mentioned before I feel I can easily fit a helix into my plans that will have 2 parallel tracks,...
Outer radius 31 inches
Inner radius 28.5 inches

I decided to glue some track down on my 'bench' in those 2 dimensions. I chose the 2.5" between tracks dimension after reading a great number of postings indicating that this seems to be adequate for basic curved tracks at these sort of radiuses.
So here are my 2 tracks that also have penciled in lines located 1" outboard of them (1" either side of the track's centerline)
 

That is a newer Walthers 85' heavyweight passenger car on the inner track. It's obivious that its forward end is just shy of that 1" clearance from the C/L.

What is not so obivious is how much does the center of this car project from the C/L of the track. Regretable my camera angle (too close to subject) hides this detail. Let me say that it appears as though the center of this long passenger car requires a FULL 1" clearance on its inboard side on this 28.5 radius curve.

How about two of those passenger cars passing one another on my helix. No problem according to this photo. )and here you can see the center overhang of that upper car barely touches the 1" clearance line at its inboard side)

Since  I'm dealing with PRR passenger cars here. i wanted to see if a GG1 could get by, (an IHC one i have).

This photo would appear to have everything OK,...but as it turns out there is considerable 'play' in its trucks that allow it to extend past that 1" clearance line. From these 2 photos we can tell that this loco requires at least 1.25" clearance on its outboard front side.

Its inboard side does NOT seem to present any problems with overhang.

And Interestingly, it still has plenty of clearance to negoiate the helix with 2.5" seperation of the 2 tracks.

...to be continued

Installing a 24" Radius Curve

I'm putting a 24" radius track on my test board today, as I may well need to utilize this tighter radius in some of my staging area, staging access,  and freight yard(s).

I'm also trying to sort out a 'focus problem' I am suddenly having with my once excellent Fuji FinePix S700 camera

good reference

That does seem to be mostly true Matt

Hmmm ...

Has anyone here read this article?

http://model-railroad-hobbyist.com/mrh2009-01/curve_insights

MRH Issue 1, January 2009

Another Hmmm.....

See Chapter 3, Figure 3-13 of Make-it run like a Dream: TRACKWORK by one Joe Fugate.

Available as an e-book for $11.99 at https://store.mrhmag.com/store/p131/ebook/run-like-a-dream-trackwork

RP-7 and parallel curves

In the methodology section TN-7 for equipment passing on parallel curves the worst case (to avoid) is car on outer curve with long wheel base and rolling stock on the inner curve with substantial end overhang. The first piece will hang in (e.g. 80' passenger car). The second piece will extend out at its front or rear corners (e.g. steam engine 4 whl trucks).

There is also a tool where you can detemine distance based on your two worst case pieces of rolling stock. You measure distance between pivot points and pivot points and corners. Then plug them in with other data.

https://www.nmra.org/sites/default/files/standards/sandrp/pdf/curved_track_center_and_obstacle_clearance_assistant_jul_2017.html

The part that vexes me is one of the inputs is center to center distance of of parallel tangent/straight track. And tighter distance allows a tighter parallel curve distance (observed by plugging in numbers, for instance 2" and 2 1/8" tangent separation with rest being example data). My intution tells me starting further a part should make no difference or be less likely to cause a hit.

UPWilly Has anyone here read

How does one go about reading that article? i keep clicking on links, but it took me around in circles and utimately back to a group of comments about the article,.....not the article itself??

The early issues

Santa Fe Passenger Cars

Decided to pick out a few of my Walthers streamline Santa Fe 85' passenger cars, since I plan on running these quite often behind a good selection of Santa Fe diesels I have.
1) On the 31" radius curve
.....everything appears to run fine. It does appear that there needs to be minimum of 1" additional 'outer clearance' for the ends of the cars, and a minimum of 1" of 'inner clearance' for the middle girth of the car.
 
Outer radius here is 31". Next in is 28.5".  The inner one with the autoracks on it is a testing minimum of 24"

2) On the 28.5" radius curve
...it appears to be very similar to the same requirement for the 31" curve above
 

3) On the 24" radius curve
...I wanted to see if these cars could negotiate a 24" radius as these tight curves might be encountered in some staging access, some yards, or a few other misc locations on my relatively small layout.

It does appear as though they can make it around these curves with those very good coupler designs provided by Walthers. It also appears as though the ends of the cars, and the middle of the car requires a wider clearance dimensions, As can be seen via that 'clearance bracket' this needs to be a minimum of 1+1/8" .
 

Too tightThoses L brackets

Too tight

Thoses L brackets with blue 3m painters tape on, they are just barely getting through. And that's when they are not moving. What about the slight wobble when running trains.

But hey if your happy running those clearances.

Phil

Too Tight

@Phil

Yes they are just BARELY getting thru with no wobble, etc.

I was really trying to determine the BARE MINIMUM dimensions possible. And thanks for reminding me of that wobble factor. 

Auto Racks on curves, Articulated Ones

Atlas Articulated Auto Carriers

At first it appeared as though these car-pairs were going to be limited to maybe 26" radius minimum due to interference at their centers. But as it went along I found they just might negotiate the 24" radius
1) On 31" Radius Curves
 

So on the 31" radius it appears as though the bare minimum clearances required are I" out from the CL of the track for either end of the cars, and 1" inboard of the track CL for the mid-girth of either car.

2) On the 28.5" Radius Curve
 

It appears that we need another 1/8" clearance (total 1"+1/8") for the ends of the cars on this smaller radius.
 

3) On the 24" Radius Curve
 

It appears here we require that extra 1/8" at both the ends of the cars, and in the mid girths.

That was actually quite surprising that it wasn't larger.

Auto-MAX

I have a couple sets of these articulated Genesis AutoMax. Since they are almost identical in length to the AutoRacks above, and are articulated as well, I did not see the neccessity of unpacking them to make measurements. I figure they will require the same clearances as those articulated AutoRacks.
 

Stand Alone Auto Racks

I regret that I did not get to test the individual AutoRack cars as they were buryied a bit too deep in my storage trailer. I would be a bit concerned about them as their axle spacing might suggest a little more overhang at their far ends.