Will My Helix Work?
HO scale helix — verdict: High Risk
24.0 in radius · 12.0 in climb · effective grade 4.20%
Helix grade, clearance, and footprint check
Will your model railroad helix actually work?
Check grade, clearance, curve drag, footprint, and build risk before you cut wood.
Diagnostic verdict
High Risk
Your HO helix is showing elevated risk. Review the grade, radius, and train type before cutting wood.
Why:effective grade is 4.20%, between the 3.5% and 4.25% caution thresholds.
Raw grade
2.87%
Curve drag
1.33%
Effective
4.20%
Your raw helix grade is 2.87%. Because this climb happens on a 24.0 inch HO scale curve, the curve drag estimate adds about 1.33%. Your train may experience this closer to a 4.20% grade.
Warnings
- The curve-compensated grade is entering high-risk territory. Test before permanent construction.
Suggested fixes
- Increase the radius if space allows.
- Add more run by increasing the number of turns.
- Reduce deck, roadbed, or safety stack height only if it does not compromise clearance.
- Shorten trains or use stronger locomotives.
- Test with your actual rolling stock before final construction.
Curve drag is a rule-of-thumb estimate, not exact physics.
Risk scores assume clean track and free-rolling metal wheelsets. Plastic wheelsets, dirty track, tight joints, and underpowered locomotives can make results worse.
Verify final clearances against NMRA RP-7.1 (formerly Standard S-7) and your actual rolling stock.
Start hereNew to model railroad helices? Read this first.
What is a helix?
A helix is a spiral ramp built into your layout that lifts trains from a lower deck to an upper deck in a small floor space. Most home model railroads with two levels use one. It is usually hidden behind scenery and looks like a stack of circular shelves with track on top.
Why this tool exists
Helices fail in predictable ways. Too tight a curve, too steep a climb, not enough headroom for tall cars, or a build that needs a strong locomotive your collection does not have. By the time you find out, the wood is cut and the upper deck is sitting on top. Fixing it means tearing it out.
What you will enter
- Scale. The size of your trains — O, S, HO, N, or Z.
- Radius. The size of the curve, measured from the center of the helix to the middle of the track.
- Climb. The total vertical distance from the lower deck to the upper deck.
- Train height. Top of rail to the top of your tallest car.
- Headroom. The air gap you want above the train.
- Material heights. Track, roadbed, and the plywood deck thickness.
- Train type. What you intend to run on it.
Not sure what some of these mean? Open the glossary.
Side view. A helix lifts trains between decks in a tight footprint.
Before you cut wood
Short reads that explain the math, the materials, and the patterns to avoid.
Why it matters
The real cost of a failed helix
Plywood, time, and a derail-prone mainline you can't easily fix. Why this check exists.
FAQ
Answers to common helix questions
What is a safe grade? Why curve drag? Where do these numbers come from?
Glossary
Helix terms in plain English
Rise per turn, easement, NMRA S-7, nolix, and what each one means for your build.
Gear
What you'll need to build it
A descriptive checklist — materials, sub-roadbed, tools — with search phrases, not affiliate links.
Examples
Three worked helix builds
HO mainline, N scale in a closet, and HO with heavy steam — each opens prefilled in the checker.