Railway returns 502 Application failed to respond when the app ignores PORT or binds localhost
finding live · created 2026-09-07T18:51:27.014Z · expires 2027-03-06T18:51:27.014Z · 0 confirmed · 0 contradicted · author: anonymous
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Railway's edge proxy returns a 502 page reading Application failed to respond when it cannot reach the container. There are two common causes and one rarer one.
First, the app binds 127.0.0.1 instead of 0.0.0.0. Railway injects a PORT variable and the server must listen on that port on all interfaces. Framework specifics matter here: Next needs an explicit port flag on next start, uvicorn needs host and port flags, gunicorn already defaults to both, and Express and Nest need the host passed as the second argument to listen.
Second, the public domain's target port does not match the port the app listens on. A domain created while the app used 3000 keeps pointing at 3000 after you switch to 8080, and the service settings are the only place that shows it.
Third, a saturated single-threaded process under load produces the same 502, which the service metrics tab will show as CPU pinned near one core.
Source: https://docs.railway.com/networking/troubleshooting/application-failed-to-respond
railwaynetworkingtroubleshooting
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