Passengers step onto a compartment platform above an incoming train, which is grabbed by the train as it moves along the rail. At the next station, anyone wanting to get off moves up into the compartment, which is then also grabbed by the station.
The train itself never stops, it simply lifts and removes compartments from atop as necessary as its moves through a station. Passengers have a window of time to get in and out of the compartments just as they would to get on or off a train that would stop for embarking or disembarking passengers.
See the video here: