The helix: gaining height in little space
Benjamin | TrackDrawer.com · published 7 November 2026
A helix is track that winds upwards over itself in turns, so you gain tens of centimetres of height on the footprint of a single circle. Each turn rises 2 x pi x radius x gradient, and it has to be high enough to clear the train on the turn below. That is why in H0 you need a radius of about 400 mm at 3 %: with Märklin C track, R2 (437.5 mm) is the smallest curve that works, and R1 is too steep.
What a helix is for
An ordinary gradient needs length, and most layouts do not have it. Climbing 70 mm in H0 at 3 % already takes 2.33 m of track (see gradients). If you want to reach a second level or a fiddle yard under the layout, you are soon talking about 15 to 30 cm of height, and that is metres of gradient. A helix coils those metres into a cylinder. The track is just as long, but it only takes up the footprint of one circle.
The price: a helix is a stretch of layout you barely see, it has to be built solidly, and it is hard to reach when something derails. It usually stands in a corner or hides behind a backscene.
The calculation
One turn is a full circle, 2 x pi x R long. If the track climbs at gradient g, it ends up 2 x pi x R x g higher after one turn. That rise has to be at least:
- the clear space above the rail of the turn below (the structure gauge),
- plus the height of the rail with its roadbed or sleepers,
- plus the thickness of the board or trackbed carrying the upper turn.
By default TrackDrawer uses the NEM 102 profile (about 55 mm in H0 without overhead wires, 65 mm with), a rail height of 6.6 mm in H0 and a 10 mm board. For the other scales the app scales these in proportion. Turn the sum around and you get the smallest radius for a given gradient: R = height per turn / (2 x pi x g).
| Scale | Clear space | Rail | Height per turn in the app | Smallest radius at 3 % | With overhead wires |
|---|---|---|---|---|---|
| H0 | 55 mm | 6.6 mm | 75 mm | 398 mm | 85 mm, 451 mm |
| OO | 63 mm | 7.5 mm | 85 mm | 451 mm | 95 mm, 504 mm |
| TT | 40 mm | 4.8 mm | 55 mm | 292 mm | 65 mm, 345 mm |
| N | 30 mm | 3.6 mm | 45 mm | 239 mm | 50 mm, 265 mm |
| Z | 22 mm | 2.6 mm | 35 mm | 186 mm | 40 mm, 212 mm |
The height per turn is the sum of clear space, rail and a 10 mm board, rounded up to 5 mm; that is what the helix assistant suggests by default. If you want a true 70 mm of clearance below the board, for tall wagons for instance, you work in H0 with 70 + 6.6 + 10 = 86.6 mm per turn and need a radius of 460 mm at 3 %.
Which curve from your box fits
With the height per turn from the table, each catalogue curve gives this gradient. The outside size is twice the radius plus the width of the track: the footprint of a round helix.
| System | Curve | Gradient per turn | Outside size |
|---|---|---|---|
| Märklin C track | R1, 360 mm | 3.32 % | 760 mm |
| Märklin C track | R2, 437.5 mm | 2.73 % | 915 mm |
| Märklin C track | R3, 515 mm | 2.32 % | 1070 mm |
| Piko A track | R2, 421.88 mm | 2.83 % | 887 mm |
| Roco geoLine | R3, 434.5 mm | 2.75 % | 907.5 mm |
| Hornby (OO) | R2, 438 mm | 3.09 % | 905 mm |
| Hornby (OO) | R3, 505 mm | 2.68 % | 1039 mm |
| Tillig TT | R11, 310 mm | 2.82 % | 640 mm |
| Minitrix | R2a, 261.8 mm | 2.74 % | 539.6 mm |
| Kato Unitrack N | R249, 249 mm | 2.88 % | 523 mm |
| Märklin Z | R2, 195 mm | 2.86 % | 406 mm |
The smallest curve of a system is almost always too steep for a helix. That is no accident: a smallest curve exists to fit a loop on a small board, not to make a train climb in it for metres on end. Remember too that a helix is one long curve, and a gradient in a curve pulls harder than the same gradient on a straight. If your curve only just stays under 3 %, you are at the limit; one size up gives long trains some margin.
An oval helix
If you have length but no depth, an oval helix helps: a straight after each half circle. The turn gets longer, so the gradient gets gentler, without a bigger radius. With Märklin C track R1 and one 24360 straight (360 mm) on each side, a turn is 2262 + 720 = 2982 mm long and the gradient 75 / 2982 = 2.52 %. With a 24188 (188.3 mm) on each side it is 2.84 %. So R1 does stay under 3 % after all, on a footprint of 760 x 1480 mm instead of 915 x 915 mm for a round helix on R2.
Building without surprises
- Even. A helix needs the same gradient all the way round. A spot that is a little steeper becomes the place where your train stalls.
- Gentle in and out. Do not let the track go from level to the full gradient in one step; a transition at half the gradient avoids a kink at the bottom and the top.
- Reachable. You need to get at every turn to rerail a wagon. Leave the centre open or make sure one side of the helix is accessible.
- Solid. The turns rest on supports all round. How to choose the benchwork for that is covered in benchwork: open frame or solid baseboard.
The helix in TrackDrawer
The Height panel has a helix assistant. You pick a curve from your system (by default the second radius, because the smallest is often too steep), then the number of turns or the height you want to reach at the end. The height per turn is already filled in following the table above; you adjust it through the rail height, the board thickness and a tick box for overhead wires. You also choose ascending or descending, clockwise or anticlockwise, round or oval, and a straight lead-in and lead-out. Those straights climb at half the gradient, as a transition.
The assistant shows straight away the number of curves, the turns, the gradient and the clearance between the turns, and warns you if the helix is too steep, if the clearance drops below the profile, or if a turn is lower than the minimum clearance of your plan. That is 70 mm by default, meant for H0; in N or Z you set it lower.
An example with C track R2 and two turns: 24 curves 24230, 5.50 m of track in the turns, a 2.73 % gradient and 150 mm of height. With the default lead-in and lead-out of one 24360 straight on each side, you end up at about 160 mm.
If you first select a piece with an open end, the helix starts there; otherwise in the middle of the view. Every piece carries its height, so 3D and the checks see the helix like the rest of your plan. The Auto-designer does not lay a helix itself; you add it. Laying a helix is part of Pro. Try out in the app which curve suits your height, and read in the figure of eight how a bridge gains height without a helix.
