We present hereafter the experimental work of building an ontology of the European Rail Traffic Management System (ERTMS) domain. ERTMS is a railway complex control system defined on the basis of publicly available specification documents, the System Requirement Specification (SRS). We will describe the methodology that we used to define an initial structure for an ERTMS ontology. The main goal of this work is to supply a first formalization of the ERTMS knowledge in order to provide the basis of a later development process i.e validating the specifications, developing the software/hardware components and finally validating the system.