Minimum Curves for Long Freight Cars & Steam Locos

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Active Member
HO scale:

I'm looking at running long cars (autoracks, etc) on 2 parallel mainline curves, (also two helix curves). I also have a number of nice big steam engines (both articulated ones and others) that I want to run on my new layout

It got me to wondering 1) what sort of minimum radius I needed to provide for these cars, 2) clearances I needed to provide between two curves, 3) clearances I needed to provide to either side of the tracks for other layout structures.

For my purposes I was very interested in two particular radius, 31" & 28.5" inches. The 31" figure is the max outer curve I can utilize for my helix. The 28.5" figure is a result of the 2.5" minimum clearance needed between the 2 tracks as determined by almost universal conscience in reading lots of forums.

I wanted to verify this CL to CL figure of 2,5", and I wanted to verify that the equipment would not have any problems negotiating the smaller 28.5" radius.

I added a third radius to my experiments, a 24" curve,...
a) to see if the equipments would negotiate this tighter curve
b) many of us with small layouts might well run into this size radius quite often
c) I might run into this size curve in my staging access track(s), freight yard tracks, etc
d) it might fill the gap of questions about the range between 24 & 28 curves.

So on my experimental table I laid down three radius tracks,...31", 28.5" , 24" . I have also included a penciled in line that is 1" to either side of the CL of each track.

BE AWARE: I am seeking to determine the absolute minimum clearances that might be required. There are likely some additional tolerances required for wobble, rocking, etc of these cars as they make these turns.
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Active Member
AutoRacks 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.


Active Member
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.

D&J RailRoad

Professor of HO
I would think that instead of asking someone else of their experience, you might experiment with your own track laying procedure so you have first hand tried and trued information. The amount of over hang of locos and rolling stock will be a bit different on different track. If you use sectional track, expect problems going into curves. If you use flex track, be sure to include transitions into the curves.
You might consider using a sheet of homasote to experiment on. Track nails can be easily pushed into it and removed. Don't go for the minimal curve radius. Try to expand beyond that as much as you can to get away from the hard turns of toy trains.


Active Member
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.


Active Member
Genesis AUTO-MAX cars

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 necessity of unpacking them to make measurements. I figure they will require the same clearances as those articulated AutoRacks.



Active Member
The advice to generate your own empirical evidence (data) is sound. There is nothing like finding out the real and practical limits for the rolling stock and its couplers and truck swing.

Note that, when testing limits, you have to be thorough and comprehensive. Trailing coupled cars around a tight curve is not the same as shoving them, where the couplers bunch up. On long passenger cars with diaphragms, you can expect vastly different results on curves that are at the practical limits for the cars on tighter curves. This will be especially bad in stub terminal yards which so many of us have, and especially those with turnouts and frogs near #5 or less.


Active Member
Very interesting observation there Crandell,...thanks.
I can recall the challenge of backing a long passenger train full of 85' foot cars into the 'double wye type' entrance to the yard of my Central Midland layout. That was a challenge !

I must keep that in mind when I try to design and operate the 3 prong access track I have in mind to service my 3 staging areas (backing into them).
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Whiskey Merchant
On the passenger cars, do you have body or truck mounted couplers? I have a string of Walthers passenger cars for my North Coast Limited all with body mounted couplers and I can back it through a double crossover and multiple turnouts with no problems. I got a string of older passenger cars from Garry with truck mounted couplers. It was a disaster trying to back them into the same spot. Sort of like trying to push a chain across a parking lot. Once the truck mounted couplers were replaced by body mounted couplers I had no problems at all.


Active Member
body or truck mounted couplers

The original set of Santa Fe passenger cars I had were the old Concor ones with truck mounted couplers that I had added the special Kadee kits to for close coupling. It was much more of a challenge with them. If I recall properly I think the wheel flanges on those cars was a bit bigger than nowadays?


Whiskey Merchant
Truck mounted couplers are fine for going forward, but when backing, they pain be a royal pain in the butt. When the couplers are body mounted, the cars are puching the cars and the trucks are more or less just along for the ride whereas with truck mounted couplers, the forces can pust fram side to side also and a derailment can be just around the corner.

The 85 foot passenger cars that I had problems with were a mixture of Riverossi and AHM of IHC ????? Not my first choice by any means but free is good. My layout is all code 70 with the exception of hidden staging (code 83) and a couple of the old Riverossi cars needed the wheels replaced because of teh deep flanges. They were plastic any way. I also have metal wheels on everything. It took a little bit of work to get the body mounted couplers all adjusted to the proper height, but once everything was tuned up, they backed through numerous turnouts as good as the Walthers cars.

I do like some of the newer Con-Cor passenger cars. I picked up a Northern Pacific shortie combine just for grins and was pleasantly suprised. The detail was excellent, operating diaphragms and the car had interior lighting. Liked it so much I went and ordered a coach. I model a rural area so short passenger trains are all that I really need.


Active Member
Stand Alone AutoRacks

I regret that I did not get to test the individual AutoRack cars as they were buried 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.

(actually from the looks of this photo it might be the mid-girth dimension that might be more critical,...those auto racks are on a 26" curve) But its hard to tell from that photo angle?
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Whiskey Merchant
If the cars are the same length, the overhang will be the same and there shouldn't be a problem. What can cause problems is when a shorter car such as a hopper and frame in the photo you posted. The shorted car will have less of an overhang which could cause a derailment problem.

Modeling the transition era, almost all of my freight cars are 40 footer with the exception of a few 50 footers so there isn't much of an overhang difference. I put an 85 foot passenger car at the head end of a train with a string of 40 foot cars and occasionally did have a derailment at one point where my minimum radius dropped to 32 inches.


Section Hand
Monday. I saw from an overpass, a CP unit train rounding a set of super elevate curved tracks. The prototype had a large sweeping curve. It was the first time to see a train here from above and impressive to have the cars leaning into the curve at speed.

My point being is to use the largest radius in your track work planning whenever you can on your layout. You'll like the improved operation and visual impact. Unfortunately, on my layout my largest radius curve is hidden in a tunnel.




Active Member
Well Type Container Cars

For brevity I just put a couple of those well type containers cars on the 24" radius track.

Looks like they fall barely within the 1" boundry lines I drew on either side of the CL of the track, certainly at their mid-girth.

DSCF2131, on 24 radius.jpg

DSCF2132, container well cars on 24 radius.jpg


Active Member
heavy Duty Depressed Center Flat Car with Tansformer Load

Tried this one out on the 28.5" radius and was pleasantly surprised that it did not overhang too much at either the ends, nor mid-girth.



(sorry for that one out of focus photo, camera acting up)


Active Member
more on that Depressed Center Flat Cat

Went back to storage trailer and picked up that transformer car again so I could put it onto my new 24" radius test track.



Just for the heck of it I decided to put it on a piece of Bachmann 18" track I happen to have setting around. This car could negotiate this track...




Active Member
Playing with Looong Transformer Car

When I was down dragging out of storage that previous transformer car yesterday, I ran across another one I had, that I had almost forgotten about. It was a loooong car I had found at a train show years ago. Actually I found 2, and sold one of them off.

(May have to replace those 'cookie-cutter' wheel sets if i ever try to run this car. But perhaps its just one of those cars that needs to sit in a powerplant scene.)

It could negotiate some of my curves,...

...even here on 24" radius curves !

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