ERTMS/ETCS is going to become the reference standard for modern railway signalling. To develop a safe and reliable Automatic Train Protection System (ATPS) based on ERTMS/ETCS, a detailed functional testing phase is needed, meeting the requirements of international railway safety standards. In this paper we deal with the functional validation of the trackside part of an ERTMS/ETCS compliant system. An extensive set of functional tests have been specified in order to thoroughly verify the system, using an innovative approach based on influence variables and state diagrams. However, such a detailed test specification requires a great amount of time and resources to be entirely executed in the real environment. Moreover, several tests need to generate abnormal safety-critical conditions that are unfeasible on the field. In this paper we describe how we overcame such problems using a specific simulation environment capable to quickly and automatically execute anomaly tests in normal as well as in degraded operating conditions.