Complex sentence: All three main UK parties propose to build a high-speed rail line to the north that would allow the cessation of 66,430 domestic flights per year, 30% of the capacity of the planned third runway.
Split into simpler sentences: All three main UK parties propose to build a high-speed rail line to the north. The predicted that it would allow the cessation of 66,430 domestic flights per year, 30% of the capacity of the planned third runway.