Deployment fails with logs: "Path: /health", "Retry window: 5m0s", repeated "Attempt #X failed with service unavailable" or "status 404. Continuing to retry...", finally "1/1 replicas never became healthy! Healthcheck failed!"

Railway deployment healthcheck polls configured path (e.g. /health) expecting HTTP 200 within timeout (default 300s). Fails if endpoint missing (404), app not ready (service unavailable), wrong port binding, or non-200 response. Expected behavior, not bug—ensures zero-downtime deploys. [Railway Docs](https://docs.railway.com/deployments/healthchecks)