As in unscheduled. An extra train is a train called to transport extra freight which is beyond the capacity of the regularly scheduled trains. Such an extra train has no rights and no schedule.
As in extra capacity. An extra section of a scheduled train is when there is more passengers or freight than can be pulled by the regular power of a scheduled train. If a scheduled train must keep to a timed schedule and has specific rights. The dispatcher will call an extra section of that train which confers all rights and schedules to the extra.