Past perfect progressive

Das past perfect progressive wird gebildet aus had + been + present participle (ing-Form).

He had been writing letters for three hours, before he went for a walk.
He had not been writing letters for a very long time, when his friend called him.
Had he been writing letters since 9 o'clock, when his friend called him?
Rob had been travelling for three months when he ran out of money.

Das past perfect in der progressive form wird dann benutzt, wenn der Verlauf einer Handlung betont werden soll, die sich vor einem Zeitpunkt in der Vergangenheit ereignet hat bzw. zu diesem Zeitpunkt noch andauerte.