Difference between "Past Progressive" and "Past Perfect Progressive"

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?

Solution 1:

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.

Solution 2:

As I understand it, past perfect prog. gives a sense of the action being interrupted as one of a longer duration than if you were to use past prog.

E.g. I had been training for two years when I won the national championship.

This is a good example because aside from that sense of duration being imbued, the structuring of the sentence makes it quite irrational to use past prog. instead