Complex sentence: When Weiden West was opened, people coming from west of Cologne could change there to the Stadtbahn instead of going via the Central Station of Cologne and use the westbound Stadtbahn to the stadium.
Split into simpler sentences: When Weiden West was opened, people coming from west of Cologne could change there to the Stadtbahn. Before, these passengers had to go via the Central Station of Cologne and use the westbound Stadtbahn to the stadium.