The figure eight: crossing or bridge?
Benjamin | TrackDrawer.com · published 1 November 2026
A figure eight is a closed loop that crosses itself once, so the train runs alternately anticlockwise and clockwise. With a crossing piece everything stays flat, and the larger the crossing angle, the shorter the eight. With a bridge you need no crossing, but you do need a gradient, and in H0 that quickly takes 2.4 to 3.5 m of length.
How a figure eight works
Put two loops side by side and let the tracks pass over each other in the middle. The train runs the first loop anticlockwise, goes through the middle, and runs the second loop clockwise. Each loop turns through more than half a circle: 180 degrees plus the crossing angle. With a 90° crossing that is two three-quarter circles, with a 30° crossing two loops of 210°.
Electrically a figure eight is an ordinary loop. The left rail stays on the left everywhere, so there is no short circuit as with a reversing loop, not even with two-rail. A related shape is the folded eight, which looks like a dogbone; it is described under the dogbone.
The crossing angle sets the length
From the crossing point the two tracks diverge at the crossing angle. Only once they are far enough apart is there room for a curve. With a small angle that takes a long way, so the eight becomes long.
In the simplest form (two circles that touch both straights) you calculate it like this:
- Width = 2 x radius + roadbed width, just as for an oval.
- Length = 2 x radius / sin(half the crossing angle) + 2 x radius + roadbed width.
The table pairs the crossings in our catalogue with the smallest radius of each system.
| System | Crossing | Angle | Loop turns | Length x width |
|---|---|---|---|---|
| Märklin C track R1 | 24640 | 24.3° | 204.3° | 4.18 x 0.76 m |
| Märklin C track R1 | 24649 | 48.6° | 228.6° | 2.51 x 0.76 m |
| Märklin K track standard radius I | 2258 | 45° | 225° | 2.63 x 0.75 m |
| Piko A track R1 | 55241 | 30° | 210° | 3.55 x 0.76 m |
| Fleischmann Profi R1 | 6160 | 36° | 216° | 3.05 x 0.74 m |
| Hornby R1 | R614 | 22.5° | 202.5° | 4.57 x 0.77 m |
| Kato Unitrack H0 R370 | 2-401 | 90° | 270° | 1.83 x 0.78 m |
| Minitrix R1 | 14958 | 30° | 210° | 1.91 x 0.41 m |
| Kato Unitrack N R216 | 20-320 | 90° | 270° | 1.07 x 0.46 m |
| Märklin Z R1 | 8559 | 13° | 193° | 2.87 x 0.31 m |
What stands out:
- The angle matters more than the scale. An N gauge eight with a 30° crossing is longer than an H0 eight with a 90° crossing.
- With C track, the choice of crossing makes 1.67 m of difference. With the 48.6° 24649 the eight is 2.51 m long, with the 24.3° 24640 more than 4 m.
- Not every system has a crossing. For Roco geoLine, Tillig Elite and Tillig TT our catalogue lists no crossing; there the bridge is the only way.
The Auto-designer works with the crossing in your stock. If the turn of a loop does not come out in whole curve pieces (204.3° is not a multiple of 30°), it closes each loop with suitable pieces from your box. Its loops also have straights between the curves, so its plans can differ slightly from the pure form above.
The figure eight with a bridge
Without a crossing piece, one track has to go over the other. One straight through the middle lies on the baseboard, the other at bridge height, and the loops are the gradients in between. In H0 TrackDrawer works with at least 72.6 mm of height: 60 mm of clearance above the lower rail, plus 6.6 mm of track and 6 mm of bridge deck, and never less than the set clearance height (70 mm by default). The default maximum gradient is 3 %. Around each crossing point the Auto-designer leaves a level stretch, and the gradient builds up gradually, so there is no kink.
A 3 % gradient over 72.6 mm is already 2.42 m of pure climb, and the loop has to achieve that between the two crossing points. A larger radius gives a longer loop and so a gentler gradient. That leads to something you might not expect: with a broader curve the eight can get shorter, because a larger crossing angle then stays within 3 %.
For Märklin C track, each time with the largest crossing angle that stays within 3 %:
| Radius | Crossing angle | Steepest gradient | Length x width |
|---|---|---|---|
| R1, 360 mm | 30° | 2.4 % | 3.54 x 0.76 m |
| R2, 437.5 mm | 30° | 1.9 % | 4.30 x 0.92 m |
| R3, 515 mm | 90° | 3.0 % | 2.53 x 1.07 m |
| R4, 579.3 mm | 150° | 2.8 % | 2.40 x 1.20 m |
| R5, 643.6 mm | 150° | 2.4 % | 2.66 x 1.33 m |
Add the edge margin: with 50 mm on each side, 10 cm in both directions. On R2 a 60° crossing angle just misses 3 % (3.1 %), hence the long eight. These are the dimensions of the shape itself. The Auto-designer only lays a figure eight with a bridge if the straights through the middle can be built exactly (to half a millimetre) from your straight track; with a different box it may therefore pick another angle or radius.
In the 3D view the upper track gets a bridge with girders, and the check in the editor keeps watching the gradient and the clearance if you change the eight afterwards.
Toy track: the bridge in the box
Wooden track works differently. The bridge comes in the box and provides the height itself; the rest lies flat on the floor. With IKEA Lillabo the bridge is two times 205 mm long, 410 mm in total, and that is exactly the length of a straight through the middle of a figure eight with 205 mm curves and a 90° crossing angle. That is why the Auto-designer finds the eight from the starter set (twelve curves, two straights and the bridge) with exactly those pieces. More ideas under Lillabo.
Crossing or bridge: which one?
- Crossing if your baseboard is short and your system has a crossing with a large angle. Everything stays flat, there are no gradients to build and the eight is quickly laid.
- Bridge if you want height in your scenery, or if your system has no crossing. Expect more length and a baseboard structure that carries the gradient.
- Two trains? On a figure eight with a crossing, two trains can only run safely if they are never at the crossing at the same time. With the bridge that is not an issue: the trains pass above each other.
Frequently asked questions
Why does the Auto-designer not suggest a figure eight with a bridge?
Usually because the gradient on your baseboard would exceed 3 %, or because the straights through the middle cannot be laid exactly from your track. A longer baseboard, a larger radius or a few extra short straights help.
Does the train really change direction of rotation?
Yes. In one loop it runs anticlockwise, in the other clockwise. On an oval it always goes the same way round, so you always see your train from the same side.
Try it yourself
Enter your track, including your crossing, and let the Auto-designer work out which figure eight fits on your baseboard, with a crossing or with a bridge.
