I have a lot of MP3 files in the format track_number artist--track name but their ID tags are not populated. Is there a utility that will run through the files and populate the tags by extracting the relevant information from the filename ?
There are plenty of tagging programs capable of doing this. I've used foobar2000 for tagging recently and it does what you want. It's using ID3V1 and APE2 tags, but ID3V2 are not supported (or need some extra plugin or manual configuration, I'm not sure) because they're located in the beginning of files and force software to re-write the whole file because of this. Other taggers people seem to like are "The Godfather" and "Tag" by Case.
