In this paper an investigation about the influence of noise on speech intelligibility (SI) inside metros under two different driving conditions (running in tunnel straight route (TS) and in curve (TC)), for female and male speakers with four voice levels (normal, raised, loud and shout) is presented. Perceptual laboratory tests were carried out where the noise of 11 metros was mixed with words of Italian language. The noise was always reproduced with the same level, while the speakers' voice amplitude differed, thus permitting to evaluate several signal-to-noise ratio (SNR) conditions to find out the SNR providing best intelligibility aboard. SI was quantified as the percentage of disyllabic words correctly understood (%WCU). Results showed, that %WCU was highly correlated with SNR. Poor intelligibility rates corresponded to “normal” voice condition. Fair SI can be guaranteed for SNR values greater than -6 dB in TC up to -3 dB in TS. Considering the use of raised vocal effort of 68,3 dB, the target noise level aboard, providing good SI, should be less than 71 dB. Besides, the results showed that the %WCU was influenced by the route conditions (driving in TC resulted in lower SI), the voice amplitude (%WCU rose with the increasing of the speech volume), the metro sound characteristics (SI scores differed in diverse metros), the gender of speakers (the voices of male speakers generally were more intelligible than female ones) and by the interaction of all the considered factors.