Minimum radius in HO
What radius does a six axle road diesel (SD40-2) need in HO scale?
Quick answer
A six axle road diesel (SD40-2) scales to 9.37 inches in HO, so it needs about a 28.1 inch radius for reliable running using the three times car length rule, or 32.8 inches for it to look right. A full circle at 28.1 inches needs roughly 60.2 inches of width. 23.4 inches is the absolute sharp minimum.
The working rule in this hobby is that minimum radius is a multiple of the length of the longest thing you run. It is a rule of thumb rather than a published standard, and it predicts reality well. A six axle road diesel (SD40-2) is 68 prototype feet, which at 1:87.1 is 9.37 inches over the couplers.
Six axle trucks need more radius than four axle ones of the same length, because the rigid wheelbase of each truck is longer. For a diesel the trucks pivot, so a locomotive tolerates a sharper curve than a rigid car of the same length. What suffers first is appearance: the body overhangs the rail and couplers on adjacent equipment pull sideways.
For context, the classic HO train set curve is 18 inches. That is sharper than this equipment's absolute minimum of 23.4 inches, so a set track oval will not run it at all. This is the single most common disappointment in the hobby: the locomotive arrives and will not go round the circle that came in the box.
On this page
Radius for a six axle road diesel (SD40-2) in HO at four standards
The four standards the hobby uses, from the sharpest curve the equipment will tolerate to the radius at which nothing is compromised. The space column is the width a full circle needs including two inches of clearance each side, which is the number that actually decides what fits in your room.
A six axle road diesel (SD40-2) in HO scale needs a 28.1 inch radius at the conventional three times car length standard, and a full circle at that radius takes 60.2 inches of width.
| Standard | Multiple of car length | Radius | Diameter | Width needed | What it means |
|---|---|---|---|---|---|
| Sharp | 2.5 times | 23.4 in | 46.8 in | 50.8 in | It will go round, and it will look like it is going round. Overhang is obvious, long cars sideswipe on parallel tracks, and some equipment will not track at all. |
| Conventional | 3 times | 28.1 in | 56.2 in | 60.2 in | The usual compromise. Almost everything sold for the scale will run reliably, and the overhang is visible but not embarrassing. |
| Broad | 3.5 times | 32.8 in | 65.6 in | 69.6 in | Long passenger cars and big articulated steam start to look right rather than merely working. This is where most people wish they had started. |
| Generous | 4 times | 37.5 in | 75 in | 79 in | Nothing is compromised and everything looks correct. It costs area, and area is the one thing a home layout never has spare. |
The car length multiple is hobby convention, not a published standard. NMRA Recommended Practice RP-11 is the standards-body document on curvature and equipment, and a manufacturer stated minimum radius for a specific model always takes priority over any rule of thumb.
The same equipment in every scale
If you have not committed to a scale yet, this is the table that should decide it. The same prototype car needs very different space in N, HO and O, and the width column is what has to fit in the room you actually have.
A six axle road diesel (SD40-2) needs a 15.3 inch radius in N and 51 inches in O, which is why the same room holds a very different railroad in each scale.
| Scale | Model length | Sharp | Conventional | Broad | Width for a circle |
|---|---|---|---|---|---|
| N (1:160) | 5.1 in | 12.8 in | 15.3 in | 17.8 in | 34.6 in |
| HO (1:87.1) | 9.37 in | 23.4 in | 28.1 in | 32.8 in | 60.2 in |
| O (1:48) | 17 in | 42.5 in | 51 in | 59.5 in | 106 in |
What happens if you use a sharper curve anyway
Nothing dramatic, usually. The train goes round. What you get instead is a set of smaller problems that add up:
- Overhang. The middle of the car swings inside the curve and the ends swing outside it. On a 9.37 inch car at 23.4 inches this is clearly visible and it is the reason sharp curves look like a toy train set.
- Sideswiping on parallel track. Two long cars passing on adjacent curved tracks can touch, which is why track centre spacing has to increase on curves. In HO the straight track spacing is about 1.79 inches and a sharp curve needs more.
- Coupler pull. On a tight curve the couplers are at an angle to each other, and the sideways component of the pull lifts the inside wheel. This is the mechanism behind the derailment that only happens when the train is long.
- Uncoupling on the curve. Knuckle couplers at a steep angle can pull apart under load, which is the fault that seems random until you notice it always happens at the same place.
The honest summary: sharp curves are a legitimate choice when space is the binding constraint, and you should make that choice knowing what it costs rather than discovering it after the track is down.
How much room does that actually take?
Radius is measured to the centre of the track, so a full circle is twice the radius plus the width of the roadbed and something to stop the train hitting the wall. These are the practical widths.
A full circle at the conventional 28.1 inch radius needs about 60.2 inches, or 5 feet, of clear width.
| Standard | Radius | Full circle width | In feet | Track length in a full circle |
|---|---|---|---|---|
| Sharp | 23.4 in | 50.8 in | 4.2 ft | 147 in |
| Conventional | 28.1 in | 60.2 in | 5 ft | 176.6 in |
| Broad | 32.8 in | 69.6 in | 5.8 ft | 206.1 in |
| Generous | 37.5 in | 79 in | 6.6 ft | 235.6 in |
A circle is the worst case. A layout that runs along a wall and turns at the ends only needs this width at the ends, which is why a shelf layout with a return loop fits where a table layout does not.
Track and equipment for a 28.1 inch radius in HO
Chosen against the numbers on this page rather than from a general list, so the recommendation cannot contradict the arithmetic above.
Atlas 500 36 inch Code 83 Super Flex Track 25 Pieces
$166.13 Specs and fitmentTwenty five lengths, which is 75 feet of track and enough for most home layouts in one purchase at the lowest price per foot.
Best for: laying a whole HO layout in code 83
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Atlas Code 100 Nickel Silver Super-Flex Track 3 foot Section HO Scale
$16.70Code 100 has taller rail than code 83, which looks less realistic and is more forgiving of older equipment with deep wheel flanges. That tolerance is the reason to choose it.
Best for: a layout running older or inherited equipment
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Atlas Code 100 Nickel Silver Super-Flex Track 5 Pack HO Scale
$39.98Five lengths of code 100, a sensible quantity for a first layout before committing to a full carton.
Best for: a first HO layout in code 100
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Atlas Code 100/83 Nickel Silver Universal Rail Joiners 48 Pieces HO Scale
$8.00Rail joiners are a mechanical connection and not a reliable electrical one. Buy plenty, and plan to solder feeders to the rail rather than trusting these to carry current.
Best for: joining any code 83 or code 100 rail
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Peco SL-110 HO Code 75 Rail Joiners 24 Pack
$9.78Peco joiners for their finer code 75 and code 83 rail. Mixing joiner sizes across rail codes is a common cause of a kink at the join.
Best for: Peco track in code 75 or code 83
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Bachmann SD40-2 DCC Sound Locomotive Union Pacific HO Scale
$215.81 Specs and fitmentA six axle road diesel with factory sound, and the SD40-2 is the single most common prototype in North America, so it is right for almost any modern era layout.
Best for: a first sound equipped road diesel
Check price on AmazonHow we chose
We did not test these products in person and we never claim to. Picks are researched from manufacturer specification sheets and instruction manuals, published NMRA standards and recommended practices, and the recurring themes in verified owner reviews. Every pick is matched to a real scale, a real minimum radius or a real current draw rather than to a price bracket, and it is placed in the tier where its capability actually belongs. Where a product is wrong for most layouts we say so on the page, and where a figure is a hobby rule of thumb rather than a published standard we say that too.
Sources
- NMRA Recommended Practice RP-11, Curvature and Rolling Stock
- NMRA Standard S-1.2, General Standard Scales
- Manufacturer published minimum radius figures on current product listings
Frequently asked questions
What is the minimum radius for a six axle road diesel (SD40-2) in HO scale?
About 28.1 inches for reliable running, using the convention that minimum radius is roughly three times the length of the longest car. The model is 9.37 inches long in HO. It will physically negotiate 23.4 inches at the cost of obvious overhang, and 32.8 inches is where it starts to look correct rather than merely working.
Will a six axle road diesel (SD40-2) run on HO train set track?
No. The standard 18 inch HO set track curve is sharper than the 23.4 inch absolute minimum this equipment needs, so it will either derail or bind. You need flex track or a larger sectional radius, which is the usual reason people move away from set track.
Is the three times car length rule an actual standard?
No, and that distinction matters. It is hobby convention, widely used because it predicts reliable running well. The NMRA publishes Recommended Practice RP-11 on curvature and rolling stock, which is the standards-body document on the subject. A manufacturer's stated minimum for a specific model is derived from that model's mechanism and always takes priority over any general rule.
How much space does a HO scale circle at this radius need?
About 60.2 inches, which is 5 feet, of clear width for a full circle at the conventional 28.1 inch radius. That includes two inches of margin each side so the train is not running along the very edge of the benchwork. A layout that only curves at the ends needs this width at the ends alone.
Does a longer car always need a bigger radius than a locomotive?
Not necessarily. For a car the body is rigid between the trucks, so length is the whole story. For a steam locomotive the rigid wheelbase between the driving wheels usually decides it, so a short locomotive with many coupled drivers can be fussier than a longer car. For a diesel the trucks pivot, which makes it more tolerant than its length suggests.
What is the cheapest way to get a broader radius?
Flex track. Sectional curves come in fixed radii and you are stuck with what the system offers, while flex track lets you lay whatever radius the space allows, including the awkward intermediate numbers. It costs less per foot than sectional track and it is the standard way layouts are built once people stop using the track that came in the box.
Researched, not professional advice. This page is compiled from published manufacturer specifications, instruction manuals, published NMRA standards and recommended practices, and owner-review consensus, not hands-on testing. Figures described as a rule of thumb are hobby convention rather than a standard, and they are labelled that way wherever they appear. Always check a radius, a clearance or a grade against your own equipment before you cut wood or lay rail, because manufacturers vary and so does what your specific models will tolerate. Any permanent mains wiring in a layout room is work for a licensed electrician to your local code.