Your layout will not close: a gap of a few millimetres
Benjamin | TrackDrawer.com · published 3 November 2026
A gap of a few millimetres in a sectional track layout almost always comes down to one of three things: a piece that is not quite the right length or angle (often a mix-up with a similar piece), small deviations that add up over many joints, or a baseboard that is not flat. Up to about a millimetre per joint the rail joiners absorb it; beyond that the track is under strain and you need to change the plan or use a short make-up piece.
How big may a gap be?
A rail joiner allows a little play, in length and in angle. By default TrackDrawer uses a tolerance of 1 mm and 0.5 degrees per joint: within that margin a joint counts as closed. If the end of a piece lies within 25 mm and 10 degrees of another but outside that tolerance, the checks report a closing error: "track is under strain", with the distance and angle.
That distinction makes sense in real life too. You will not notice half a millimetre. Three millimetres you can close with some pushing, but then the track is bent somewhere else, a joint opens up or a turnout sits askew. That is where your train will derail later, and you will look for the fault in the wrong place. How that plays out at a turnout is covered in train derails at the turnout.
Cause 1: a piece that looks almost the same
The most common cause is a swapped piece. Many systems have straights that differ by only a few millimetres or centimetres, and curves with the same angle but a different radius. A few examples from our catalogue:
- Piko A-Gleis has the G239 (239.07 mm) and the G231 (230.93 mm), and also the G119 and G115.
- Roco geoLine has straights of 200 and 185 mm.
- Minitrix has straights of 54.2 and 50.0 mm.
- On Märklin C track R1 and R2 are 77.5 mm apart, but both exist as 30 degree curves.
One wrong straight in an oval immediately gives a gap the size of the difference. One curve of the wrong radius in a half circle gives a gap and an angle error. When in doubt, lay the pieces side by side or read the part number underneath. With mixed or second-hand stock it pays to sort everything first; see giving your old box of track a second life.
Cause 2: small deviations that add up
Sectional track is designed so that a turnout and matching curve on one side reach exactly as far as the straights on the other. That nearly works, but not always to a tenth of a millimetre. This is the difference per turnout end in our catalogue:
| System | Turnout and matching curve reach | Straight side (turnout plus make-up) | Difference per end |
|---|---|---|---|
| Märklin C track | 360.08 mm | 360.00 mm (with 24172) | 0.08 mm |
| Märklin M track | 359.76 mm | 360.00 mm (with 5106) | 0.24 mm |
| Piko A-Gleis | 470.00 mm | 470.00 mm (with G231) | 0.00 mm |
| Minitrix | 158.30 mm | 158.40 mm (with 14906) | 0.10 mm |
| Fleischmann piccolo | 222.58 mm | 222.00 mm (with 9101) | 0.58 mm |
| Hornby | 335.23 mm | 336.00 mm (with R600) | 0.77 mm |
| Kato Unitrack N | 248.98 mm | 248.00 mm (with 20-040) | 0.98 mm |
| Peco Setrack N | 174.50 mm | 174.00 mm (with ST-1) | 0.50 mm |
| Roco Line | 574.91 mm | 575.00 mm (with G1 and G1/2) | 0.09 mm |
For most systems that is negligible. With Kato Unitrack N, Hornby, piccolo and Peco Setrack N it adds up: a passing loop has two turnout ends, so with Kato N there is a 1.96 mm difference between the two tracks, with Hornby 1.54 mm, with piccolo 1.16 mm and with Peco Setrack N 1.0 mm. Put two passing loops in a row and it doubles. Spread such a difference over several joints rather than forcing it closed in one place. The TrackDrawer Auto-designer does the same: it spreads such remainders over the joints of the passing loop.
What else matters with two parallel tracks is covered in track spacing.
Cause 3: the base
A board that is not flat, a carpet that gives, or a gradient that is not quite even shifts the track a fraction. On the floor, a layout that closed yesterday may have a gap today. On a permanent layout gaps often come from strain in the wood or cork. Lay the track loosely on the board first, check that everything closes, and only then fix it down, working from the middle outwards.
How to close the gap
- Find the wrong piece. Count the pieces on each side and compare with your plan. Often the fault is in one piece.
- Spread small differences. A rail joiner gives up to about a millimetre per joint. Do not push everything into one joint.
- Use a short make-up piece. Many systems have short straights: Märklin C 64.3 mm, Piko 61.88 mm, Minitrix 17.5 mm, Tillig TT 21.3 mm. A gap of a few centimetres usually closes neatly with one.
- Use flexible track. If you have a permanent layout and a system with flex track, cut a piece to size. More on that choice in sectional track or flexible track.
- Change the plan. A different combination of straights on the opposite side often solves a structural gap better than forcing it.
How TrackDrawer helps
TrackDrawer shows every closing error as a message with the two pieces, the distance in millimetres and the angle error. Open ends appear as orange connection points. Tap one and the gap closer searches your own stock for a run of at most six straight and curved pieces that closes the gap within the tolerance. If fixed pieces will not do and you have flexible track in your stock, it suggests a curved flex track instead.
Frequently asked questions
Why does my oval close on the floor but not on the baseboard?
On the floor the pieces shift until they lie without strain; on a board with screws they cannot. Lay the track loosely, let it settle and only screw it down afterwards.
Can I just push a 2 mm gap closed?
On a temporary layout, yes, but you move the strain somewhere else. On a permanent layout it is better to find the cause or spread the difference over several joints.
Check it yourself
Redraw your layout in TrackDrawer or let the Auto-designer lay a plan from your stock: you will see at once whether everything closes, and where it does not.
