El tren viajaba cuando de repente llegó a la parada.
Explanation:
El tren viajaba cuando de repente llegó a la parada. This means: the train was traveling when suddenly it arrived to the stop. The sentences combines the imperfect with the preterite. This is because an action was taking place in the past, in this case "el tren viajaba" when some other action interrupted it, in this case "llegó". This is the typical combination of those tenses. The imperfect is used for the action that gets interrupted by the precise action in the preterite.
