In a Communication Based Train Control System (CBTC), a central zone controller server (ZC) exchanges signaling messages with on-board carborne controllers (CC) inside the trains through a wireless technology. The ZC calculates and sends periodically to each train its Limit of Movement Authority (LMA), i.e. how far the train can proceed. A CC triggers an emergency break (EB) if no message is received within a certain time interval to avoid collision. Clearly, it is not desired to have an EB due to signaling messages losses (called spurious EB) and not to real risks for the trains. Quantifying the rate of spurious EBs and predicting correctly CBTC system performance are hard tasks with important industrial relevance. This work aims at filling this gap using simulation to better predict CBTC system performance and avoid extra provisioning before deployment. A typical CBTC system implementation for metro by Alstom Transport is considered. New ns-3 modules (CBTC protocol, Video traffic generator, multi-channel scanning mechanism, 3D antennas patterns) are developed and a piece of existing code is enhanced. The simulation is also used to investigate the dimension of the radio access networks in a realistic environment (specific modems and access point antennas, radio frequencies, train and track models), another aspect also ignored in the previous literature. Last, our approach can be useful to validate some analytical works.