Automatic Passenger Counting (APC), or accurate estimation of the number of passengers in public transport, is crucial for optimizing transit operations, improving passenger experience, and supporting data-driven transport planning. Recently, thanks to the increase of popularity of machine learning and AI, two main APC technologies seem to emerge: Wi-Fi-based systems and optical camera systems. The firsts have gained popularity due to their low cost, but they often suffer from accuracy issues caused by signal interference, multiple devices for each person and more importantly the randomisation of addresses. As for camera systems, many commercial APC systems promise very high accuracies, but in real world conditions the accuracy plummets due to poor lighting, bus vibrations, and struggles to count people in crowded situations.