I always interpreted “that notion” to mean to watch his speed. I.e. - SLOW DOWN!!!
Also, while I may be in the minority, I think the GD never meant to imply that Casey Jones, the actual real railroad engineer mentioned in the song, was the guy using cocaine. I always interpreted this as (of all things from this band) an anti-drug song. Cocaine was just coming into vogue in the early 1970’s, and it seems to me that the song is comparing cocaine users and their reckless amp-ed up behavior to a railway car careening towards an ugly crash. And the legend of Jones’ tragic end was a well-known folk legend at the time, so it made for a good metaphor.
Maybe it’s not preaching complete abstinence (as I think Jerry was busted for coke sometime after the song became popular), but perhaps moderation, or caution. Coke certainly didn’t mesh well with the relaxed, laid back, easy-going vibe of the Dead’s music. That’s more weed, hashish and shrooms music.