Knowing the exact number of passengers among the city bus fleets allows public transport operators to optimally distribute their vehicles into the traffic. However, interpreting overcrowded scenarios, at rush hour, with day/night illumination changes can be tricky. Based on the visual trackingby-detection paradigm, we benefit from video stream information provided by cameras placed above doors to infer people trajectories and deduce the number of enterings/leavings at every bus stop. In this way a person detector estimates the location of the passengers in each image, a tracker matches detections between successive frames based on different cues such as appearance or motion, and infers trajectories over time. This paper proposes a fast and embeddable framework that performs person detection using relevant state-of-the-art CNN detectors, and couple the best one (in our applicative context) with a newly designed Siamese network for real-time tracking/data association purposes. Evaluations on our own large scale in-situ dataset are very promising in terms of performances and real-time constraint expected for on-board processing.