Railway does not provide a traditional private IPv4 address for your services. The private domains you see (e.g., service.railway.internal) are for internal communication between your own Railway services only, not for sharing with external providers. What you need for whitelisting with an external payment service is a static outbound IP, which is available on the Pro plan. Once enabled, it gives you a fixed IPv4 address that your API will use for all outgoing traffic, which you can then provide to your payment provider.