Complex sentence: The extent of the rapid transit system varies greatly between cities, with several transport strategies: in larger metropolitan areas the underground system may extend only to the limits of the inner city, or to its inner ring of suburbs with trains making frequent station stops.
Split into simpler sentences: The extent of the rapid transit system varies greatly between cities, with several transport strategies. Some systems may extend only to the limits of the inner city, or to its inner ring of suburbs with trains making frequent station stops.