This paper proposes a new Line of Sight (LoS) probability model for metro carriage scenarios to evaluate radio propagation for the future Fifth Generation (5G) Wireless Communications Systems in the Millimeter Wave (mmW) bands. The new model has been defined after 36 measurement campaigns. For each campaign a high precision 3D scenario with a random distribution of passengers and different positions of TX has been used. A comparison between the proposed model and ITU, WINNER A1 and WINNER B3 LoS probability models has been made. As a result, it is shown that the proposed model is the most suitable for this scenario and considerably minimizes the Mean Square Error (MSE) compared to the other models.