Thread a few months ago https://news.ycombinator.com/item?id=41547008
I attended a lecture from Conway where he described this approach -- but I don't recall him calling it FRACTRAN. I don't even recall what the particular example computed, but the program was around a dozen fractions.
This was probably at the end of the 70s while I was a Maths undergrad at Cambridge.
I was there. I drew the artwork for the poster for the talk that got pinned up on noticeboards around the univrsity. I met Conway a handful of times, mostly at DAMTP, and he was always interesting, always with new ideas.
Posted at the time: https://news.ycombinator.com/item?id=23142232