No, it's not equivalent to to 'had been working'. I'm not sure what the exact grammatical explanation is, but verbs used this way (as part of a subordinate clause?) mean something like: 'by means of' 'as a result of'. Other example would probably be better than my attempts to explain: Walking quickly, he reached class well ahead of time. Speaking loudly, she was heard even in the back of the auditorium. Saving every penny for years, they were able to retire comfortably.