Complex sentence: Southern operated a pilot for The Key between Brighton and Seaford in 2010 as these stations are only operated at by Southern trains (with the exception of Brighton) which was a success and so in 2012 it began to gradually expand across the network.
Split into simpler sentences: Southern operated a pilot for The Key between Brighton and Seaford in 2010 as these stations are served only by Southern trains (with the exception of Brighton). This was a success and so in 2012 it began to gradually expand across the network.