Glossary

Helix terms in plain English

A working vocabulary for helix planning. Each definition is written for builders, not engineers — the goal is to make the checker's output legible, not to be exhaustive.

Helix

A spiral ramp of curved benchwork used to lift trains from one deck to another in a small footprint. Common in two-deck and multi-deck home layouts.

Rise per turn

The vertical distance between successive levels of the helix. Equal to train height plus headroom plus track, roadbed, deck thickness, and safety margin. Drives the grade math.

Train height

Top of rail to top of the tallest car you plan to run. The number you measure with a ruler against your actual rolling stock. Modern double-stack containers and auto racks are the limiting cases for HO and N.

Headroom (above train)

The air gap between the top of the train and the bottom of the deck above. Half an inch is tight and unforgiving of deck sag; one inch is comfortable. Not the same as rise per turn — headroom is just the air, not the whole stack.

Raw grade

Rise per turn divided by track length per turn, expressed as a percentage. Ignores curve resistance.

Curve drag

Added resistance caused by the train constantly turning while climbing. Wheels scrub, flanges contact, and locomotive adhesion drops. The classic HO rule of thumb — credited to John Allen — adds 32 divided by the radius (in inches) to your raw grade percentage.

Curve-compensated (effective) grade

Raw grade plus curve drag. The grade your train will actually feel on the helix.

Easement

A gradual transition between tangent track and a curve, used to reduce sudden lateral force on long cars. Most useful at the entrance and exit of the helix, not inside it.

Nolix

A layout that gains elevation gradually along the mainline itself instead of using a hidden spiral. Requires long, narrow rooms, but avoids the helix's reliability problems entirely.

Roadbed

The material between the deck and the track — typically cork, foam, or homasote. Adds a small but real height to the clearance stack.

Sub-roadbed

The plywood or hardboard surface the roadbed sits on. The helix deck itself is sub-roadbed.

NMRA RP-7 / RP-7.1 (formerly S-7)

The current NMRA Recommended Practice for track centers and obstacle clearances, with scale-specific clearance diagrams for bridges, tunnels, and overhead obstructions. The former Standard S-7 was reorganized into the RP-7 family — hobbyists still use the legacy 'S-7' name in conversation.

Effective radius

On a multi-track helix, the radius of the innermost track. Always tighter than the nominal centerline of the helix structure and the one that usually drives risk. The tool computes a verdict for each track separately when double track is on.

Track spacing

The center-to-center distance between two parallel tracks on the same deck. NMRA RP-7.2 covers the official numbers; in practice HO is about 2 to 2.5 inches, N about 1.5 inches. Used as the difference between inner and outer track centerline radii in a double-track helix.

Footprint

The outer diameter of the helix benchwork, plus any aisle space needed to reach inside it. The space the helix actually occupies in the room.

Safety margin

Extra height added on top of the clearance stack to absorb construction error, deck sag, and unexpectedly tall equipment. Typically a quarter inch in HO scale.