The oval: the simplest layout and how to expand it
Benjamin | TrackDrawer.com · published 1 November 2026
An oval is a closed loop of two half circles with two straights of equal length between them. It is as deep as twice the radius plus the width of the track, and as long as that figure plus one straight. You almost always expand it on the straights: first a siding, then a passing loop or a small yard, and with a deeper baseboard a second loop.
How an oval is built
Each half circle turns the track through 180 degrees. With most systems that means six curves of 30°, so twelve for the whole oval. Not every system works that way: Fleischmann Profi track has 36° curves on R1 and R2 (five per half circle), Kato Unitrack H0 has 22.5° curves (eight per half circle). So count your curves per system, or check the system page to see which curves belong to which radius.
The outside dimensions follow from three numbers:
- Depth = 2 x radius + roadbed width.
- Length = depth + the length of one straight.
- Baseboard = outside dimensions plus a margin to the edge on every side; TrackDrawer starts a new plan with 50 mm.
Two examples the Auto-designer actually laid with a typical box of Märklin C track:
- On 180 x 90 cm: "Oval R1", 22 pieces and 4.11 m of track. Each straight is 924.9 mm (four times 188.3 mm plus one 171.7 mm). The outside dimensions are 168.5 x 76 cm.
- On 240 x 120 cm: "Oval R3", 32 pieces and 5.69 m of track, with straights of 1225.4 mm. The outside dimensions are 229.5 x 107 cm.
On an oval the two straights lie exactly twice the radius apart. What is left inside is the place for a station, a siding or a piece of scenery.
Big curve or long straight?
On a given baseboard you have a choice: a tight curve with long straights, or a broad curve with shorter ones. Perhaps surprisingly, the broad curve gives slightly more track on the same baseboard. Every extra millimetre of radius costs four millimetres on the straights (two per side), but adds 2 x π millimetres in the curves. On balance the loop gains about 2.3 mm of track per millimetre of radius.
On 240 x 120 cm with C track and a 50 mm margin, the numbers are:
| Radius | Depth of loop | Straight (max.) | Total track |
|---|---|---|---|
| R1, 360 mm | 760 mm | 1540 mm | 5.34 m |
| R3, 515 mm | 1070 mm | 1230 mm | 5.70 m |
That is why the Auto-designer picked R3 for the plain oval on this baseboard: with the straights from the box, each side came to 1225.4 mm and the loop to 5.69 m. The broader curve also runs more smoothly, and long coaches look better on it.
There is a price, though. The straights are where turnouts go. A passing loop in C track needs at least 72 cm of straight, and every siding, yard or station also takes its room from those sides. If shunting is your main interest, keep the curve tight and the straights long; if you mostly want to watch trains run, choose the big curve.
Expanding step by step
The Auto-designer builds its variants the same way you would at home: first the loop, then one element per layer on a side that is still straight. On 240 x 120 cm with the typical box of C track (per radius a full circle plus four curves, eight each of six straight lengths, three left and three right turnouts, four counter curves, three buffer stops) that produced this ladder:
| Step | Variant | Pieces | Track | What is added |
|---|---|---|---|---|
| 1 | Oval R3 | 32 | 5.69 m | only curves and straights |
| 2 | Oval R3 with siding | 37 | 6.69 m | 1 turnout, 1 counter curve, 1 buffer stop |
| 3 | Oval R3 with passing loop | 34 | 6.54 m | 2 turnouts, 2 counter curves |
| 4 | Yard on R3 | 40 | 7.31 m | 2 turnouts, 2 sidings |
| 5 | Station on R3, plus 2 sidings | 52 | 8.80 m | 5 turnouts, 1 passing loop, 3 sidings |
| 6 | Double track R3/R2 | 60 | 10.83 m | a second loop on R2 |
| 7 | Double track R3/R2, plus siding and yard | 69 | 11.97 m | second loop and 3 turnouts |
What the table shows:
- A siding is the cheapest gain. One turnout, a counter curve and a buffer stop add just over a metre of track and a place to park a train.
- A passing loop turns running into a timetable. Two trains can wait for each other, on the same baseboard and with only two extra turnouts.
- Turnouts stack well. With five turnouts the oval becomes a station with a passing loop and three sidings: 3.1 m more track than the plain oval.
- The second loop is the biggest jump. Double track almost doubles the track length, but needs a second set of curves and a baseboard deep enough for the outer loop. More on that in double oval.
Expanding without tearing up
The nice thing about an oval of sectional track: a turnout often fits exactly where a straight was. In C track the turnout is 188.3 mm on the straight route, the same as straight 24188. With Piko A track the turnout is 239.07 mm, the length of the G239, and with Roco geoLine 200 mm, the length of the G200. You take out the straight, put in the turnout, and the oval stays closed. The branch then gets a counter curve so the new track runs parallel to the oval, and the straight route gets a compensation piece if a second turnout follows.
That way the layout grows in the order you buy track: first one turnout with a siding, later the second turnout that turns it into a passing loop.
Other shapes from the same oval
The oval is also the starting point for almost every other shape:
- Dogbone. Squeeze the oval in the middle and you get two turning curves with two tracks close together. Read the dogbone.
- Figure eight. Let the two straights cross each other, with a crossing or over a bridge. Read the figure eight.
- Polygons. Spread the curves over three, four or six corners. On 240 x 120 cm the Auto-designer laid a "Diamond on R1, plus 2 sidings" of 55 pieces and 7.61 m: more track than the oval R3 with a siding, on the tight curve.
Which shape gets the most out of your baseboard depends on its size. For the most popular size it is worked out in an H0 layout on 180 x 90 cm.
Frequently asked questions
How big does my baseboard need to be for an oval?
At least as deep as twice your smallest radius plus the roadbed width, plus a margin on both sides. With C track R1 that is 760 mm, or 86 cm with a 50 mm margin. The length is up to you: every extra centimetre becomes straight.
Isn't an oval boring?
Only if it stays bare. With one siding you already have something to shunt, with a passing loop you can run two trains. The oval remains the loop on which a train simply keeps running while you watch.
Try it yourself
Enter your track and your baseboard size and let the Auto-designer show which oval fits, and which additions your turnouts still allow.
