Complex sentence: An extension of the branch was planned in 1871--72 to connect to south via Dedworth and Ascot but, despite reaching an advanced stage of design and property purchases plus the construction of a possible station building, this was never completed and abandoned completely by 1914.
Split into simpler sentences: An extension of the branch was planned in 1871--72 to connect to the south via Dedworth and Ascot. Despite reaching an advanced stage of design and with some property purchased plus the construction of a possible station building, the plans were never completed and were abandoned completely by 1914.