Past perfect continuous

I had been saving money for two years.

The continuous tenses emphasize the ongoing nature of the activity. Think of them as a variation of the standard perfect tenses. Here are the four perfect tenses:

present perfect: I have lived in Madrid for two years.

present perfect continuous: I have been living in Madrid for two years. => emphasizes the ongoing nature of the activity

past perfect: I had lived in Madrid for two years.

past perfect continuous: I had been living in Madrid for two years. => emphasizes the ongoing nature of the activity

With verbs of duration like live and work, both forms are correct and the difference is subtle. With many other verbs, the standard form emphasizes a completed result (I have read the book), while the continuous form emphasizes the ongoing activity (I have been reading the book).