Reversing loop
Work in progress: This section is still a work in progress — content is being added and revised continuously.
What is checked? Whether the plan contains a route over which a train returns onto the same track in the opposite direction — the polarity reverses (classics: balloon loop, wye, certain double-slip arrangements). A plain oval is not a reversing loop.
Why does it matter? On two-rail DC systems, plus and minus rail meet at the reversing loop → short circuit. This is not a planning error — reversing loops are operationally valuable! — but they need insulated gaps and a reversing-loop module (digital) or a polarity switch (analogue). On three-rail/stud-contact systems (Märklin) the problem does not exist electrically.
How to fix it: Don’t rebuild — plan ahead: insulated gaps on both sides of the loop section, budget a reversing-loop module, make the section long enough for the longest train.
Numbers & defaults: notice level on three-rail systems.