Complex sentence: It was never a high speed lie: its winter business was carrying freight and local passenger traffic over the Mendips, but in the summer season the S&D came into its own, carrying a heavy service of Saturday holiday trains from northern towns to Bournemouth.
Split into simpler sentences: It was never a high speed line, its winter business was carrying freight and local passenger traffic over the Mendips. In the summer season it came into its own and carried a heavy service of Saturday holiday trains from northern towns to Bournemouth.