Subway station indicating / Subway station indicating lamp

When the subway arrives, you get to know that there have already been a large number of people in that carriage, and you have to rush into it for you do not have enough time to get on another.
Maybe people can invent a device to install in that station to detect the number of people in those carriages so that the number of passengers can be delivered to people waiting at the next subway station. There should be a projection above the corresponding door in the subway station, and let carriages’ information projects onto the ground in order to inform people of visual information about the carriages.