9/12/11: Rerouting

What does it mean when a train gets "rerouted?"  It's rare that I get kicked off a morning train, unlike the oft-run Coolidge Corner expresses, but we were all tossed off the train at Park Street today as the train got rerouted.  Granted, I only have a one-stop walk to Government Center, but I find it odd that they decided this train was needed somewhere else.  There isn't exactly a lot of outbound traffic at this time of day.