This never-stopping Chinese bullet train concept was unveiled back in April:
The passengers at a station embark onto to a connector cabin way before the train even arrives at the station. When the train arrives, it will not stop at all. It just slows down to pick up the connector cabin which will move with the train on the roof of the train.
While the train is still moving away from the station, those passengers will board the train from the connector cabin mounted on the train’s roof. After fully unloading all its passengers, the cabin connector cabin will be moved to the back of the train so that the next batch of outgoing passengers who want to alight at the next station will board the connector cabin at the rear of the train roof.
The video is helpful for visualizing this: