Those aren't necessarily tracks but services. 3 services can share one mainline and several stops. Makes (dis)boarding easier for longer distances if passengers don't have to change trains after they get out of the peninsula, plus 3 services hitting the same stations means 3x as much frequency along that corridor. Someone going Jacksonville to Miami can pick between the Chicago, LA or New York route and someone going from Miami to LA can just hop into the LA route and stay there until they arrive without changing trains