Complex sentence: The mainline station was opened by the London Chatham and Dover Railway company with the name St. Paul's in 1886, and the Underground station was added in 1870 with the arrival of the Metropolitan District Railway.
Split into simpler sentences: The mainline station was opened by the London Chatham and Dover Railway company with the name St. Paul's in 1886. The Underground station had been opened in 1870 with the arrival of the Metropolitan District Railway.