The European Rail Traffic Management System/European Train Control System (ERTMS/ETCS) is an innovative standard introduced to enhance reliability, safety, performance, and interoperability of trans-European railways. In Level 3, the standard replaces fixed-block safety mechanisms, in which only one train at a time is allowed to be in each railway block, with moving blocks: a train proceeds as long as it receives radio messages ensuring that the track ahead is clear of other trains. This mechanism increases line capacity, but relies crucially on the communication link: if messages are lost, the train must stop within a safe deadline even if the track ahead is clear. We develop upon results of the literature to propose an approach for the evaluation of transient availability of the communication channel and probability of train stops due to lost messages. We formulate a non-Markovian model of communication availability and system operation, and leverage solution techniques of the ORIS Tool to provide experimental results in the presence of multiple concurrent activities with non-exponential durations. © Springer International Publishing Switzerland 2015.