Track planning
Track Centers and Clearance Chart
Quick answer
HO needs at least 3.37 in of railhead-to-railhead clearance for standard equipment on a double-stacked or upper level, scaled from a 19 ft prototype clearance plus a 0.75 in subroadbed allowance. N needs 2.18 in and O needs 5.50 in, and double-stack container cars need more than any of these baseline figures.
Track centers and clearance answer two different questions that both come from the same idea, prototype separation scaled down to a model. Track centers set how far apart two parallel tracks sit on the same level, scaled from a 13 ft real-world separation. Clearance sets how much vertical space a lower level needs under an upper level or a bridge, scaled from a 19 ft prototype clearance with an added allowance for the physical thickness of the subroadbed itself. Both numbers come from the same math the track plan size calculator uses.
On this page
How far apart should two parallel tracks sit?
Track centers are scaled down from a 13 ft prototype separation between adjacent tracks, the standard industry spacing that keeps two trains from side-swiping on tangent track.
HO track centers are 1.79 in; scale that same 13 ft prototype spacing down to O and it grows to 3.25 in.
| Scale | Track centers (scaled from 13 ft prototype) |
|---|---|
| N | 0.98 in |
| HO | 1.79 in |
| O | 3.25 in |
How much vertical clearance does a lower level need?
Clearance is scaled from a 19 ft prototype overhead clearance, plus a fixed 0.75 in allowance for the thickness of the subroadbed itself, which the prototype figure alone does not account for.
O scale needs 5.50 in of clearance between levels, more than double the 2.18 in that N scale needs for the same double-deck layout.
| Scale | Minimum clearance, railhead to railhead |
|---|---|
| N | 2.18 in |
| HO | 3.37 in |
| O | 5.50 in |
NMRA S-7 covers clearances. Double-stack container cars, tri-level auto racks and other tall equipment need more than this baseline figure, which represents standard-height rolling stock.
Why do double-stack container cars need more than the baseline figure?
The clearance figures above are scaled from a standard prototype clearance envelope, sized for conventional single-height freight and passenger equipment. A double-stack container car is taller than that envelope by design, since it exists specifically to carry two containers stacked on top of each other, and a real double-stack route is built to a taller clearance standard than a conventional freight route for exactly that reason. A model layout running double-stack equipment on a lower level needs to add height above this chart's baseline clearance, not use it as-is.
How do track centers and turnout number interact on a ladder?
Track centers set the fixed target spacing a turnout has to reach; turnout number, multiplied by that spacing, sets how far down the ladder the turnout travels before hitting it. The full worked table for that relationship, across N, HO and O, lives on the turnout number chart. A wider scale's larger track centers is one of the two reasons a given turnout number produces a physically longer turnout in O than in N; the other is the larger overall scale ratio itself.
What you need to actually measure and hold these figures
Every number on this page is a measurement you have to reproduce on the benchwork, and two of them are easy to get wrong. Track centres have to be held consistently along a whole parallel run, and clearance has to be measured railhead to railhead including the subroadbed the upper track sits on, which is the allowance people forget.
HARDELL Digital Caliper 6 Inch with Large LCD
$22.99Reads inches and millimetres at the press of a button, which matters because track centres are quoted in inches and scale gauges in millimetres. A ruler is not accurate enough for a clearance you only get one chance at.
Best for: checking clearance and track centres against this chart
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Atlas Track Planning Template HO Scale
$9.25A drawing template of real track geometry, which is how parallel track centres get held consistently along a run rather than drifting by eye.
Best for: laying out parallel track to a consistent spacing
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Woodland Scenics HO 2 foot Track-Bed Strips 12 Pack
$17.94The roadbed under the upper track is exactly the allowance that has to be added on top of the clearance figures above. Knowing its thickness before you build the crossing is the difference between a train fitting and not.
Best for: the subroadbed allowance in a clearance calculation
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Atlas 500 36 inch Code 83 Super Flex Track 25 Pieces
$166.13 Specs and fitmentFlex track lets a parallel run be laid at whatever centre spacing the chart calls for, rather than the fixed spacing a sectional double track system imposes.
Best for: parallel track at your own chosen centres
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 Standard S-7, clearances
Frequently asked questions
What is the difference between track centers and clearance?
Track centers measure horizontal spacing between two parallel tracks on the same level. Clearance measures vertical spacing between a lower track and whatever sits above it, whether that is a bridge, a scenery feature, or an upper deck. Both scale down from real prototype separation figures, but they solve different physical problems on a layout.
Is NMRA S-7 the standard that covers layout clearances?
Yes, NMRA S-7 addresses clearances for the hobby. The figures on this chart apply the prototype clearance and subroadbed allowance the standard is built around, but always check S-7 directly, or a manufacturer's stated equipment height, before finalizing a tight double-deck design.
Do curved sections need wider track centers than straight track?
Yes, in practice. Cars overhang farther on the outside and inside of a curve than they do on tangent track, so many published spacing guides widen the baseline track centers on curves, particularly sharper ones. The baseline figures on this chart apply to straight, tangent track.
How much clearance does a helix need between loops?
A helix needs at least the baseline clearance figure for its scale between each loop, plus the same subroadbed and equipment-height considerations as any other double-deck clearance. Because a helix stacks many loops in a small footprint, clearance there is usually the tightest design constraint on the whole layout.
Why is O scale clearance more than double HO clearance instead of proportional to the scale ratio?
Clearance scales from the same 19 ft prototype figure across every scale, so the ratio between scales in feet stays constant; the reason O looks like more than a simple ratio jump from HO is the added fixed 0.75 in subroadbed allowance, which does not shrink with scale the way the prototype-derived portion of the number does.
Does track center spacing change between N, HO and O for the same reason clearance does?
Mostly yes, both scale down the same category of prototype figure, 13 ft separation for track centers and 19 ft clearance plus a fixed allowance for vertical space. Track centers do not carry an added fixed allowance the way clearance does, since there is no equivalent to subroadbed thickness in a horizontal spacing measurement.
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.