Yo hablaba por teléfono cuando mi tren llegó a la estación.
Explanation:
Yo hablaba por teléfono cuando mi tren llegó a la estación. This means: I was talking on the phone when the train arrived. "Hablaba" is in the imperfect, because it refers to a past action that was taking place when other action in the past occurred or interrupted it. "Llegó" is in the preterite because it's a past action that was started and completed in the past.
