Tight turns

Looks to be about a 9 degree curve (637 foot radius). 

That scales out to about 144 inch radius in O scale. 

Wow!  What looks tight on the real thing is still huge on the model.

  I recall from reading somewhere 109" in 1:48 was the tightest that a mainline had. Which & where, and where I read it, I can't recall.

I've seen specs for the F-3 as having a minimum radius of somewhere between 250 to 275 feet (depending on publication). 250 feet scales to 62.5 inches, 275 feet becomes 68.75 inches. So 2-railers aiming for 60-inch minimum radius are not too off from prototypical minimum (at least for an F-3). Our SoCal favorite steamer SF 3751 (4-8-4) can handle 360 foot radius, which scales to 90-inch radius.

I remember when Atlas expanded their line of 2-rail track to 54-inch radius, I was hoping they'd go a notch or two farther to 58.5 then 63. Guess the sales of 49.5 and 54 didn't support expansion.

The REALITY is, you have to make do with the space you have regardless of what is out there in the real world. That said, if you want REAL tight curves, check out:

http://members.trainweb.com/be...ndloco/crrnjbxt.html

Simon

90s,(+inside loop) two 100s into stubs & 104s (+outside loop) and built for 36' cars. 

Center to center vs outside rail 0-48 to 0-54 ?

What do you mean by 0-48 and 0-54?    Generally in 3 rail when they mention 0-xx, it means xx diameter, not radius.    e.g. 0-48 is supposed to be 24 inch radius which is very tight for 2 rail.    

In 2 rail we generally just refer to curve radius without the "o".    so 48 inch curves generally means 48 inch radius curves or 96 inch diameter for a turn back.    and I think many of us measure center line to center line.
48  inch radius by the way is a very workable curve for 2 rail.    It is sharp by some standards, but most equipment except modern long (70-80 ft) freight cars will go around it.    Also most cars can be modified on the underside to make the curve.    I have run full length Walthers pullmans on 48 inch radius.    Just keep the underside details clear of the truck swing.    It does not look the best but works.   

I think any 4 axle diesel model with easily hand 48 inch radius and sharper.   Steam engines up to medium size generally do.    I have a Pennsy USH mikado with all wheels flanged, and it will do it, but it has to be laid smoothly.    6 Axle diesels will often do it but you have to test them.   A lot of the new stuff from MTH in 2 rail probably does.   I have some Sunset E7s and they do.   

My minimum is 60" radius, measured to inside rail.  My SP articulated coach has an extra 1/16" between carbodies just to make a 74" radius.  None of my 5- coupled steamers will put up with less than 70".

But then, I like accurate models and big steam.

I built my rather simple 2-rail layout using curves of 108 inch radius (with easements), specifically so that I could run large articulated steamers and have as little overhang as possible. Some people will object that I wasted a lot of valuable real estate by doing this, but my pleasure is to just to run a train from time to time, and the big engines look pretty nice on such a wide-radius curve (although still very sharp by the standards of the real thing). I could have packed in a lot more track and switches by using a minimum radius of 60 inches, but I've had several overly complex layouts in the past and this time (probably my last layout) I wanted some really different. Just my personal preference.

I run 6-axle locomotives and 89' flat cars around 49.5" curves with no problem.

Food for thought, easing the gauge just a little makes a huge difference in the curves.  If you are using flex track you will find that generally one rail is pretty well stuck and the other pretty loose.  The loose rail can be 'pushed' wider and then super glued to hold the spacing (superglue and zap kicker).  If hand laying then simply widen the gauge a little.  Works for me anyhow.  Russ

For those of us into full prototype radius in minimum space:

Alco (300 & 600HP) boxcabs, EMD NW1/2, SW1s and SD7/9 (SD7/9 not coupled to anything, turning radius only) could go around 59 degree curves (100 foot radius - about 24" in O scale)

Older SW 7/9/600/1200 series switchers were rated at 39 degrees (150 foot radius) - GP7/9.  More modern SW1500 and up are limited to 200 foot radius.

GE 44 Ton, Alco FA1/2 - RS1/2/3 and EMD F3/4/5/7/9 - GP9 - E7/8/9 were limited to 275 feet.

In contrast some SD40-2s could handle 200 foot depending upon the trucks, 2 coupled units were limited to 250 feet and coupled to 50 foot car the minimum radius is 338 feet..

SD70M-2s are listed at 200 feet - not coupled to cars.

ES44 - 275 feet

Big steam definitely likes big curves.    And I think newer stuff is more sensitive than the older USH and MG brass.

I do have a fairly new 10-coupled PRR N1 from Sunset 3rd rail last run.    It has no problem with 52 inch on my main and will also handle 48 inch on my branch.    All wheels are flanged.    I have an old MG 2-10-4 (PRR J1) and it does like 52 inch, even with blind center drivers.    It goes around, but under protest and again, no kinks.

I have also run a recent Sunset N&W Y3 (or Y6) that belonged to a friend.    It easily handled 48 inch radius but as you say, quite an overhang.   

As for Large curves, I could have build a loop with very large curves in my 23x47 space.    But my primary running interest is operations.   I don't get much from just seeing them run around.    I like to switch industries and make up trains following a pattern.    I have switchlist generator that tells me where the cars go, and then I and my buddies run the RR to deliver them.