From this resource I find two situational explanations of Past Progressive:
"Interrupted action in the past." with an example
She was reading a book when the light went off, had a shower and left.
And for Past Perfect Progressive:
"To show that something started in the past and continued up until another action stopped it." with an example:
They had been playing soccer when the accident occurred
For me both are kind of interruption. And both were continued up until another action stopped it. Are those bad examples? Or I am missing something? Or second part of Past Progressive ("had a shower and left.") is crucial?
Best Answer
The difference is subtle, but there is one.
In She was reading a book when the light went off the focus is on the activity at the time of the interruption.
In They had been playing soccer when the accident occurred the focus is on the activity in the time before the interruption.
But to all intents and purposes the grammatical forms are interchangeable in such contexts.
As an aside, the second part of the Past Progressive sentence "had a shower and left" is not only of no importance to the choice of construction, it is also somewhat bizarre.