For several years, the vulnerability of Critical Infrastructures (CIs) to cyber-threats has been limited, since they were mostly isolated systems, using proprietary protocols. Nowadays, CIs are increasingly threatened by external attacks: the use of off-the-shelf components is common, they have become interconnected, and sometimes also connected to the Internet. This problem is exacerbated by the recent trend towards the adoption of wireless connectivity and mobile devices, which is gaining interest also in this domain. One of the main challenges is to quantify the impact that external attacks may have on the infrastructure, and ensure that its dependability and safety requirements can still be fulfilled. In this paper we focus on the ALARP system, which protects workers on the railway infrastructure using distributed mobile terminals, and evaluate the impact of two attacks to the communication infrastructure. In performing such analysis, we experiment with a new method, which combines a stochastic model of the system with a model of the attacker, and quantifies the impact of specific attacks on precise safety and availability metrics.