Wasn't drug testing the result of the Ricky Gates fiasco/tragedy?
I think I was grandfathered in as an engineer about 1992, and I know there was random drug testing by then. It did take the FRA a while to shift random testing procedures closer to time on duty versus at end of shift to enforce the alcohol provisions.
https://law.justia.com/cases/federal/appellate-courts/F2/928/167/452334/
For 1978 and that era, I would think it would be more likely to be an alcohol problem, though drugs were out there.