I'm glad someone else looked, too. That was exactly what I initially wrote. Then I went to Lewisohn, "The Beatles Recording Sessions" page 48, which has a quote from George Martin,
"...And it was very difficult to find a three-track machine that worked. Eventually we found an old one which we prevented from overheating by having a vacuum cleaner to reverse, blowing cool air onto it. The first thing Geoff (Emerick) and I did was transfer the original tape onto 24-track and worked from there."
Of course that was a 10+ year old memory at that time, but that was the quote I went by. I didn't go back and look at the original liner notes (I should have). Do they say 16-track?
Right. The Beatles Rock Band game used Cedar Retouch to separate the instruments. The results, which were good enough for a game, were less than professional quality (IMHO). It always sounded to me like they took whatever they got out of Retouch and said, "good enough", rather than treating that as step one.
What I based my comment on, rightly or wrongly, was that Cedar Retouch is credited for the remasters as a touch-up tool.
http://www.prismsound.com/music_recording/studio_news.php?story=0179
So, I believe forensic audio software was used but not to separate instruments. I believe Beatles 1/1+ probably used forensic audio software, particularly the early live tracks, but it's really hard to tell with the way the 5.1-channel mix was done as extended stereo and I haven't found any credits that indicate the software was used. I think I saw a Giles Martin quote that said he used forensic audio software but it wasn't quite good enough for anything but bass instruments (I think that was the quote).
It will be interesting to see which software was used for the separations on the Live album. At least they already credited the engineer this time.
Thanks for the great clarifications/corrections!
Andy