Complex sentence: The brake fins in the launch section are mounted to keep fast-moving trains from moving backward into the station; however, the fast-moving train being pulled forward caused an unexpected stress on a number of fins, which bent them forward.
Split into simpler sentences: The brake fins in the launch section are mounted to keep fast-moving trains from moving backward into the station. However, the fast-moving train being pulled forward caused an unexpected stress on a number of fins bending them forward.