Shared Variables is a new feature in Railway that allows developers to define secrets in a centralized place and consume them across multiple services, making it easier to ship microservice and monorepo projects. It can be accessed from Project Settings or the Variables tab within a service's settings, and shared variables can be promoted from service-level variables to project-level shared variables. This feature exposes shared variables to Railway's template engine under the `shared.*` namespace, allowing developers to create complex references to these variables in their code, such as referencing a GraphQL URL with a variable that includes the domain and path of the shared variable.