Kato Unitrack · N & HO · the closure solver
Why won't my Unitrack loop close?
A gap RailMind cannot close is usually one of five things, and only one of them is about millimeters. This page is the solver's reasoning in plain words.
1. The enabled curves cannot make the turn
Every curve turns the track by a fixed angle — 15°, 30° or 45° in N, 22.5°, 11.25° or 10° in HO — and a straight turns it by nothing. The turn a gap needs is fixed the moment both ends exist: it is the difference between their headings, and no amount of straight track changes it. If that turn is not a whole number of the steps your curves make, no arrangement of them — any count, any order, any number of extra loops — can ever arrive anti-parallel to the other end.
That is why the app says, when it applies, “Can't close with your enabled curves: this gap needs 22.5° of turning, but they only turn in 45° steps — try a smaller-radius curve or a turnout.” The degrees are the real numbers for your gap. The fix it suggests is the right one: a curve with a finer arc, or a turnout's diverging leg, adds a step the lattice was missing. (The Auto‑route panel's Curve radii your trains can run chips let you exclude radii your trains cannot run; an excluded radius is not in the pool the solver reasons with.)
2. The straights do not match
An oval closes only when its two straights are equal. Turnouts are the usual reason they are not: a #4 is 126 mm through its main route and a #6 is 186 mm, so the side that carries them is longer than the count of S248s suggests. The passing-siding oval balances 126 + 3 × 248 + 126 on one side with 3 × S248 + S124 + 2 × S64 on the other — 996 mm each. Miss the S124 and the loop is 124 mm open with the curves perfectly fine.
3. The 33 mm rule is not quite 33
Parallel Kato track sits on 33 mm centers, but a turnout does not get you there for free. A #4 turnout diverges at R481 over 15°; follow it with the opposite-hand R481-15 curve and the track is parallel again — at 32.779 mm, not 33. The 0.22 mm is real geometry and is absorbed by rail-joiner slop on the table. A #6 with its R718-15 reverse curve comes out at 48.931 mm. The double crossover (20-210) and RailMind's parallel-copy tool use a clean 33 mm, so a parallel built with a turnout and one built with a crossover disagree by about 0.2 mm.
RailMind treats ends within 2 mm as joined — the editor's seam tolerance, and about what real Unitrack has in joiner play — so the 0.22 mm closes on its own. That 2 mm is fixed. The Auto‑route panel's Max gap slider (0–4 mm) is a different setting: it only sets how far a fix from Close loop or Auto-route may land from the other end.
4. The piece that would close it is not in the catalog
Kato makes an expansion track (20-050, 78–108 mm, variable) and flex track (21-000). RailMind leaves both out on purpose: a piece with no fixed geometry would let a plan close on screen with a length nobody can buy. Published plans sometimes close with an “EXP” piece, and that gap cannot be reproduced piece for piece in fixed track. If a plan you are copying relies on one, the honest answer is to rework the run, not to fudge the seam.
5. One turnout cannot make a reverse loop
Connecting a single turnout's two exits into a loop asks for two things at once: the run must come back to within about 16 mm of where it started, and it must arrive anti-parallel to the diverging leg — 195° round. But a loop that returns to its start has turned a net 360°, which leaves it parallel to the main. 360° is not 195°, and no sequence of catalog pieces makes it so. A working reverse loop uses two adjacent turnouts and joins their diverging legs with a teardrop; RailMind's Reverse Loop · Small Locos is that plan.
What “closed” means
RailMind's HUD reads Closed when a plan has no open ends, not when it is a loop. A bumper caps an end, so a switching plan whose every track ends at a bumper reads closed; a turntable's stalls are termini, not loose ends. The Reverse Loop reads two open ends because its main is a lead, not a loop — that is the plan, not a fault.
See it in a plan
Two #4 turnouts and a teardrop — the loop one turnout cannot close. Reverse Loop · Small Locos →
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