When the CTA inherited the elevated system in 1947, much of the system still operated "on sight", which depended on the motorman's comprehension of and compliance with safe following distances and the rules governing them.

I think I recall reading that this "on sight" operation continued for quite a while at least on some of the elevated track including the downtown loop where most of the lines converge. I would assume a few bad accidents prompted the upgrades to automated signaling and stop trips.