In public transport, acoustic intelligibility is extremely important as passengers have to be able to understand the different messages delivered by PA systems. An estimate of this acoustic property can be carried out by determination of STI or RASTI indexes. In this paper, a simulation based on the ray-tracing method and use of a commercial software is proposed (ODEON). While this method is usually relevant in large rooms, its implementation in a narrow room, like a train, with many internal obstacles may lead to erroneous results (diffraction effect, interference etc.). A case study is presented here and compared with measured values during commissioning. Limits and first order parameters for this type of study will consequently be discussed. In conclusion, basic guidelines for favourable intelligibility with regards to different simulations on several types of rolling stock will be suggested.