Ensuring railway interoperability in Europe implies that the functional specifications have a common understanding by all the stakeholders. One solution to guarantee that point is the formalization of the specification. Under the French project called 'PERFECT', we aim to formalize railway specifications and validate various systems in order to determine, using software tools, the compliance between ERTMS and National railway requirements. The big challenge is to provide a user-friendly formalization with verification tools. In this paper, we present our preliminary exploration of the use of Hierarchical Colored Petri Net (HCPN) in modeling ERTMS/ETCS (European Rail Traffic Management System/European Train Control System) functional specifications. The purpose of this contribution is to point the benefits in using HCPNs, which are widely used among railway operators, for understanding the specification and analyzing system safety features. The present work focuses on modeling the functional terms of establishing a communication session between the on-board equipment and the Radio Block Center specified in the System Requirements Specification (SRS).