It is important to remember that both simple past and past progressive, as we often colloquially refer to them, are actually the same tense:
- simple past with a simple aspect: action occurance
- simple past with a progressive aspect: action over a progression
It gets interesting... You have to consider the conjunction "when" or preposition "while" (among others), or any other adverbial phrase.
when: refers to an action percieved as a point in time
while: automatically emphasizes the duration of the action, so you'd see why the "progressive" aspect might be useful here to add that extra emphasis.
Simple + Simple
The past tense with a "simple" aspect just means that the action was carried out with emphasis on the point at which that action occurred.
I listened to music while I ran.
(I listened to music at some point while running--a general remark or fact, remains unemphasized. Note that I used "while", which might seem counterintuitive. This is why it is perhaps better to use the following example, because "while" hints at a timespan)
Simple + Progressive
English can emphasize actions as they occur over a timespan using the "progressive" aspect, which is just your typical be+verb+ing scheme.
I listened to music while I was running.
Here I have an action over a timespan "running" and at some unspecified point during the "run" timespan, I listened to music.
Progressive + Progressive
What if I combine two verbs with the progressive aspect?
I was listening to music while I was running.
This is very specific, because it emphasizes two actions over the same timespan (using while as a conjunction instead of "and" forces this), because the tenses are the same.