Complex sentence: A key year in the history of Mottingham was 1866, when the area became connected to the railway system with the construction of Mottingham station (then known as Eltham station).
Split into simpler sentences: A key year in the history of Mottingham was 1866. This is when the area became connected to the railway system with the construction of Mottingham station (then known as Eltham station).