Probably. But I am sure setting enough handbrakes to hold it if that failed was also in the manual, even if not always done.
Certainly it is now...
What could possibly go wrong with leaving an unlocked, giant, 27 to 37 year old internal combustion engined vehicle idling on a steep driveway overnight?
Also, if tank cars now rent for $100/day, 72 of them is ~$300/hr. It would seem that one could hire someone to drive them on their way for less than $300/hr rather than let them sit overnight.