The development of railways applications must be supported by rigorous methodologies and trusted tools, able to cover the whole development process, from system design to its verification and validation. This paper describes a tool supported model-based methodology for testing railway Interlocking systems (IXL). The testing process starts with the specification of abstract test cases, that are independent of a specific railway plant, in a Controlled Natural Language (CNL). The abstract test cases are then specialized to specific railway plants and executed on simulated plants. The methodology also allows for the automated generation of test cases to complement manual test case specifications by the experts. The overall approach is supported by the tool TOSCA that allows to edit, execute, and report on the results of the tests. The approach is currently used by experts for testing IXL systems in the Italian railways.