FAQ
Helix planning, answered
These are the questions builders ask most often before committing to a helix design. Answers are deliberately plain. If you want the underlying math, the checker exposes raw grade, curve drag, and effective grade as separate numbers.
What is a helix?
A helix is a spiral ramp built into a model railroad layout that lifts trains from one deck to another in a small footprint. It is usually hidden under the upper deck and used to connect staging, multiple operating levels, or a return loop. Almost every two-deck or multi-deck home layout uses one.
What is a safe grade for a helix?
Most experienced builders aim for an effective grade of 2 to 2.5 percent, with 3 percent as a practical ceiling for typical freight operations. Above 3.5 percent, expect trouble pulling longer trains, especially with light or older locomotives. The checker flags anything above 4.25 percent as high risk.
What is curve-compensated grade and why does it matter?
A train climbing on a curve experiences more drag than the same grade on tangent track. The wheels scrub against the rail, flanges contact more often, and the locomotive's adhesion drops. The checker adds an estimated curve-drag percentage on top of your raw grade so the verdict reflects what the train will actually feel.
What radius should I use for an HO helix?
Most HO helices use 24 to 30 inch radius. Tighter than 22 inches starts limiting what equipment will track reliably — long passenger cars, articulated steam, and 89 foot intermodal flats are particularly sensitive. Larger radius reduces drag and improves tracking, but increases the footprint.
What radius should I use for an N scale helix?
N scale helices typically run 13 to 18 inch radius. Below 11 inches you are in tight territory for anything longer than a 50 foot boxcar. N has more radius headroom relative to footprint than HO, which is one reason it is popular for multi-deck designs.
Why don't you support metric units?
Version 1 is inches only to keep the math, the presets, and the explanations consistent. Most North American model railroad track, lumber, and reference material is in inches. Metric support is a real candidate for v2.
Why don't you have a locomotive database?
Locomotive performance varies enormously by individual unit — drive mechanism, motor, weight, age, tuning, and how clean the wheels are. A general database would give false precision. The train-type selector is intentionally coarse; the real test is your actual locomotive on a workbench mockup of the planned grade.
Where do the curve-drag numbers come from?
They are scale-specific rule-of-thumb coefficients. The HO value of 32 traces back to John Allen — the classic formula adds 32 divided by the radius (in inches) to your raw grade as an estimate of effective grade on the curve. The other scales are calibrated against widely-published model railroad planning literature and common builder experience. They are estimates, not measured physics — a well-tuned locomotive may outperform the prediction; a tired one will underperform it.
How is rise per turn different from headroom?
Headroom is the air gap between the top of your tallest car and the bottom of the deck above. Rise per turn is the total vertical distance between successive levels — train height plus headroom plus track, roadbed, deck thickness, and your safety margin. The grade math uses rise per turn, not headroom alone. The tool keeps train height and headroom as separate inputs so you do not have to add them in your head.
Do I add easements to my helix?
If radius is tight, yes — easements between tangent and the helix entrance reduce sudden lateral force as cars enter the curve. Inside the helix, the radius is constant, so traditional easements do not apply. The benefit comes at the transitions.
Should I plan for double track?
Double-track helices are common but add complexity: the inner track has a tighter radius and steeper effective grade than the outer, so it carries more risk. The tool supports double track directly — switch the Track count toggle to Double, enter inner and outer centerline radii, and you get a separate verdict and effective grade for each track. The headline verdict is the worse of the two.
Can I trust the verdict for my actual build?
Treat it as a gate, not a guarantee. A clean verdict means the math is not the constraint. A risky verdict almost always indicates a real problem. Either way, build a test loop with your worst-case train and weakest locomotive on the planned grade before laying permanent track.
What clearance does the NMRA recommend?
The NMRA publishes scale-specific clearance dimensions in its Recommended Practices RP-7 (Track Centers and Obstacle Clearances) and RP-7.1 (Tangent Track Centers and Clearance Diagrams). These replace the older Standard S-7, which is still the name many hobbyists use in conversation. The open-clearance defaults in this tool are informed by RP-7.1 plus a small comfort margin. For a final build, verify your numbers against the current RP-7.1 spec and your tallest piece of rolling stock.
What is a 'nolix' and is it better?
A nolix is a layout design that gains elevation gradually along the mainline itself, without ever folding back into a hidden spiral. It avoids the helix entirely. It uses more wall length per inch of climb than a helix does, so it only works in long, narrow rooms. If your room shape allows it, a nolix is often more reliable than a helix.
Sources and further reading
The facts and numbers on this page are drawn from published model railroad references and the current NMRA documentation. The tool's curve-drag estimates trace back to John Allen's rule of thumb; grade and clearance guidance reflects current builder consensus.
- Compensation of grades for curvatures — Model Railroad Hobbyist Forum
Discussion of the 32/R curve-drag rule and its origin with John Allen.
- Calculating Model Railroad Grades — Free Model Railroad Plans
Explains raw vs effective grade and the curve-drag formula in plain terms.
- Helix Selection Guide — Precision Model Railroad
Practical HO and N scale helix grades, radii, and clearances from a kit manufacturer.
- Helixology for N scalers — Model Railroader
N scale specific guidance on radius and grade trade-offs.
- NMRA Standards and Recommended Practices index
Official index. Confirms the former Standard S-7 is now part of the RP-7 family.
- NMRA RP-7.1 — Tangent Track Centers and Clearance Diagrams (PDF)
Current scale-specific clearance diagrams. The authoritative reference for overhead clearance.
External links open in a new tab. The tool itself is independent — no affiliate relationships with any of the above.
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