For the record,  I set an MTH H-8 on the inside 60" track and a GG PRR full length passenger car on the outside 64" track and measured a drill rod 5/16" clearance.  Your brand may vary.

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Eternity is a long time to have been wrong.


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AGHRMatt posted:

That's darn close (probably won't clear a Big Boy). Did you push on the boiler to gain maximum swing-out?

I was going to say the opposite. That looks like plenty of room. I'm sure I have less than 1/4" with a PSC Class A on my inner curves. I did tests to make sure before design, though. I can imagine the scale passengers screaming as the running board aims directly at them on the curve.

I think you are right about the Big Boy. Test, test, and test.

FWIW, the MTH Big Boy from the 2017 Catalog has more swing than the Lionel Vision Line Big Boy.  We had the club layout working for the VL-BB, and I took the MTH BB up, BAM!  Hit the tunnel support with at least 1/8" overlap, the VL-BB has about 1/8" clearance at the same place.

I don't take the MTH BB up any more.

On Metro-North Railroad going westbound out of New Haven station, the five (straight) tracks are in a cut with stone retaining walls (about 30 feet high) on either side and there are four overpasses within a half mile of the station. At the south side beneath the overpasses, the clearance between the (load-bearing) retaining wall and the car side narrows to between one foot and, at most, eighteen inches. I'm amazed that the New Haven Railroad was willing to build it this close. But they didn't dare do it on curves...


The old swing of the curve test ... NMRA has standards for such things .....but I think they were only considering 1/4" scale .......put a 17/64ths  piece on the same curve ...the gap narrows test with the biggest thing you have then add a bit .... 

As a collector of vintage O scale ... I run both 1/4" and 17/64ths ....all clearances are done with a Mini-Scale  17/64ths Pullman ...biggest thing to roll the rails .    Here are photos  to compare gap with 1/4" VS 17/64ths 

Cheer Carey 


clearancesquarter inch cars17 64ths cars



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The NMRA standards are greater than what you are allowing but I haven't checked them recently.

When I designed my curves, I eased the outer track out an extra 3/8" so I have 4 3/8" centers on curves.  I did this on all curves less than or equal to 72"R.  On wider curves, 78"R and 82"R, I did not enlarge the center-to-center distance.  I call the extra 3/8" "peace of mind".


Ed, smart move for your class 1 main line RR.  The NMRA clearance on 72" radius is 4 5/16" center line.  However, I sold off all my class 1 equipment due to limited space and had planed several years ago to build for branch line operation.  The big engines and cars I now have I kind of backed into, they are / were only meant for display.

Being a branch line operation almost all my r.o.w. is single track.  This dual stretch is a staging area approach track..

Eternity is a long time to have been wrong.

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