Probably not in '79, but from what I read somewhere on UP's site, they've apparently had them for quite a while.
They also have track-condition sensors on trains -- I've seen those at work; caused a loose rail to get fixed within a couple days (I lived across the road and had noted the different sound when it got loose) rather than when it caused an incident. You couldn't tell there was a problem by looking at it.
My impression is that everything about rail is hideously expensive, and busted stuff is even more expensive, so they already spend more than the average effort. I was reading about some relatively new regulations that cover stuff UP has done for decades, but now have to be done according to some federal rulebook rather than with an eye to what needs doing. UP said this won't improve safety (it doesn't actually fix anything) but will increase costs (by about 60% in that part of their operation). Which seems about par for the course with incident-driven "OMG Safety!" regulations.