FA-67154 / Railway interlocking logic / Member archive

Signal lamp proving and aspect degradation: auxiliary filament fallback · case 04

A signal degrades although its auxiliary filament is working.

Member previewVariant 4 · 3 implementations · 8 checks per implementation

Case contract

Aspects need lamps: G needs G; YY needs Y1 and Y2; Y needs Y1; R needs R. A lamp is lit if its main or auxiliary filament works. If the requested aspect cannot be lit, show the next more restrictive aspect that can (G, YY, Y, R); if even R cannot be lit the signal is DARK. alarm is raised if the signal is DARK or any lamp used by the shown aspect runs on its auxiliary filament. degraded means shown != requested.

Why this case matters

Interlocking logic decides whether trains may be given authority; a wrong decision at this point either grants unsafe movements or strands traffic.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
regression: green on auxiliary filament	`{"alarm": false, "degraded": true, "shown": "YY"}`	`{"alarm": true, "degraded": false, "shown": "G"}`	Failed

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