As mentioned, there used to be a definate divide between those trainmen working in a yard versus those working local or road assignemnts. It was common to find signs designating the switching limits. Within those limits, the yard crews handled all switching. Remember seeing photos of a steam or diesel engine struggling to switch out a bad order out of a train or double over in a yard - even though the train had its road power and road crew on?

Something else to consider is when two operations were merged, certain employees often retain "prior right" to bid on certain jobs. For instance, the most junior PE employee could outbid the most senior SP employee for a "PE" job. I heard a story that was something like that if at some point if a PE job had been held 30 days or more by an SP employee, it became an open job. The former PE employees were careful to bump an SP employee of a PE job before it became open, until there just weren't enough former PE employees left to fill all the PE jobs.

I believe Amtrak still has prior right train crews. This only applies to the crews that first came over from the freight carriers when Amtrak started running their own crews. For example, a former ATSF employee could not be outbid by a former SP or UP employee on the run from LAX to San Diego - which is on former ATSF track.

Most of the recent mergers seem to have dovetailed or merged seniority rosters rather than use prior rights.

Now - what was the difference between the "Cornfield" and the "Bull Ring". I remember being where all the engines were stored dead in the late 70s/early 80s, and there was a small yard that had most of a yard ladder or lead of hand-thrown double-slip switches.