An interesting idea that I've seen several people write up about is the idea of a non-stop train that one would leave by getting onto another train that would stop. The stopping train would then either pull into a station immediately, or cruise about town to lower-traffic destinations.
The primary advantage of this is that the main-line train does not have to stop, ever. This saves energy and runs faster. People going to destinations further down the line are not slowed by people getting off on the station where the train is now, and the train can be refuled and repaired at the end of the line.
The primary disadvantages are space and safety. If someone should be a little too slow in disembarking, suddenly the stopping train pulls away, dropping the slow passenger onto the tracks at speed. Ouch. Space-wise, this means all train stations need to have an extra set of tracks that moves next and paralell to the main trains, for a fairly long distance. (However long it takes to disembark the train, reembark the new passengers, plus two extra minutes for safety's sake all times speed to produce a distance.) This distance can be reduced if the main train slows down, but that lessens the advantages given above. (Only a little bit.)
This probably won't happen for safety's sake. That one slow person getting dropped on the tracks would probably die, and most societies aren't willing to kill for efficiency's sake. (Also, I'm pretty sure at least one person would fall out per trip. Some people are very clumsy.)