Complex sentence: About 80% of the heat comes from the operation of the trains, 15% from other equipment, and 5% from people--it is calculated that tube passengers account for about 56 gigawatt-hours of heat energy emitted on an average year.
Split into simpler sentences: About 80% of the heat comes from the operation of the trains; 15% from other equipment; and 5% from people. It is calculated that tube passengers account for about 56 gigawatt-hours of heat energy emitted on an average year.