Complex sentence: On older rolling stock, with non-mechanical slam doors, the guard could lean out of the door's window and acknowledge the staff this way; however, with the introduction of electronic systems, train designers started using electronic door systems that had no windows.
Split into simpler sentences: On older rolling stock, with non-mechanical slam doors, the guard could lean out of the door's window and acknowledge the staff this way. However, with the introduction of electronic systems, train designers started using electronic door systems that had no windows.