502 "Application failed to respond" — app starts successfully but proxy can't reach it

Question

**Platform:** Railway (Docker builder) **Runtime:** Node.js 20 / Express on Debian Bullseye Slim **Database:** SQLite via Prisma

**What's happening:** My backend deploys successfully and the logs confirm the app is running:

```
SAR backend running on port 3001

```

But every request through the Railway-provided domain returns a 502 "Application failed to respond" error.

**My setup:**

* `EXPOSE 3001` in Dockerfile
* `PORT=3001` set in Railway Variables
* `targetPort = 3001` in `railway.toml`
* App binds to `0.0.0.0` explicitly: `app.listen(3001, '0.0.0.0', ...)`
* `startCommand = "npx prisma db push && node dist/index.js"` in `railway.toml`

**What I've tried:**

1. Set `PORT=3001` as an explicit Railway Variable
2. Added `targetPort = 3001` to `railway.toml [deploy]`
3. Changed `app.listen()` to explicitly bind to `0.0.0.0`
4. Tried baking startup into a `start.sh` script using `exec node` (PID 1) instead of `startCommand`
5. Confirmed `prisma db push` succeeds in deploy logs before the app starts

**Question:** Is there something else Railway's edge proxy requires beyond `targetPort`, `EXPOSE`, and `0.0.0.0` binding? Could `startCommand` running as a shell chain (`&&`) prevent Railway from detecting the correct PID or port? Is there a way to view what port Railway's proxy is actually trying to connect to?