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?