Service names must be unique across environments within a project, so if you have a service named "admin" in one environment (production, for example), you won't be able to name another service "admin" in another environment (such as development or staging)

That's weird as I've definitely got two services with the same name in both staging and production.

A service that runs across multiple environments will have the same name across all environments. But if you have a service that is only present in one environment, you can't give another service in another environment the same name.

Ah sorry, think I understand now- a service needs to synced from one environment to another for railway to treat it as the same service?

Yes, that's correct.

There's more on the process for syncing environments in the Docs.