One idea I can think of off hand is to have a detachable car system such that the train picks up a passenger car waiting at the station while dropping off a car as well. For example the last car of the train could be designated as the one that will be detached, so passengers that want to get off at a station would move into that car. One problem with this idea, though, is that it would greatly limit the number of people a single train could take on or put off at a single station and require shuffling passengers around the train between stations.
Another version is to have each person in their own bubble that can be individually handled and shuffled around the train automatically. This isn't really feasible because of extra resources that would be required for each passenger capsule, extra automation in the train, etc. It's an idea, though.
What are your thoughts on this? Is there another form of mass public transit that would be even more efficient, possibly handling both longer faster routes (trains) and shorter last-mile routes (buses)?