"High Grade" vs. "Low Grade". "Highline" vs "Lowline" parallel railroad lines in RR engineering terms. - Trains Magazine

Years ago I thought low-grade line meant subpar line. Like a low-grade whiskey or low-grade cut of meat. "Grade" as in quality.

I took it to mean the line was not in good repair. So what I imagined the low-grade line to be was where the slow drag freights ran, the low-priority merchandise.

So I basically had it bass ackward.