Learn Spanish (Latin America): El tren salió a tiempo.

In Spanish, the set phrase for "on time" (meaning punctual) is always a tiempo. Directly translating the English "on" to en is a common trap, but en tiempo isn't used this way.

Why is salió in the preterite tense here rather than the imperfect (salía)?

The preterite salió is used because the train's departure was a single, completed event. If you used the imperfect salía, it would change the meaning to "the train used to leave on time" (a repeated habit in the past) or set a background scene.

If I wanted to say "the train left," could I use the verb dejar instead?

No, dejar means "to leave" only in the sense of abandoning something or leaving an object behind (like leaving your bags at the station). For departing or physically going out of a place, you must always use salir.

If the train wasn't on time, how would we use this lesson's grammar to say "The train took an hour to leave"?

You would say El tren tardó una hora en salir. You use the conjugated verb tardar (tardó), add the amount of time (una hora), and then use en + the infinitive verb (salir).