The deployment of the European Rail Traffic Management System (ERTMS) in Europe will be mandatory on the major corridors, but this process will be long and expensive. Industry needs faster roll-out and a reduction in cost in order to obtain the certification and authorization to put equipment into service. New lab-testing tools for European Train Control System (ETCS) validation based on automatic testing could accelerate the process. In this work, we propose a co-simulation platform relaying on two existing tools: the ERTMS simulator that implements only the functional sub-system (ETCS) of ERTMS; and the OPNET simulator that allows modeling the whole telecommunication subsystem (GSM-R) from the Physical layer to higher layers, including the Euroradio interface. We describe a co-simulation protocol that determines how the results obtained from each simulator are reused as input in the other in order to take into account the impact of the functioning of one subsystem on the other. In this way, the problems related to radio propagation impairments, network load and electromagnetic interferences can be taken into account in ERTMS evaluation. Furthermore, some actors of the railway industry think about moving towards emerging technologies like LTE (4G Long Term Evolution). This platform will also allow evaluation of prospective technologies in the framework of ERTMS scenarios before costly real-world implementation.