It is the central London railway terminus for suburban First Capital Connect services from Hertford , Welwyn Garden City and Letchworth . Until March 2009 it was also a terminus for trains on the Thameslink line , also run by First Capital Connect . It is the central London railway terminus for suburban First Capital Connect services from Hertford , Welwyn Garden City and Letchworth and was , until March 2009 , a terminus for trains on the Thameslink line , also run by First Capital Connect .