There was a short time in the 90s that two zone service on Metra cost less than CTA fares. At stations such as Jefferson Park, where one had the option of taking either Metra or the L, commuter rail ridership spiked.
In some service areas, such as the IC catchment, there has also long been some understanding of tiered service based upon cost. The commuter rail service was considered premium, and usually ridden by white collar office types, whereas blue collar workers of lesser means would take the a bus/streetcar or the L.
We also, historically, need to look at how the Dan Ryan, Congress, and Kennedy extension L lines evolved CTA to more of a feeder service system with many bus routes rather than one reliant upon walk up to the L and direct bus services to downtown that it was in previous eras, once CRT, CSL, and CMC were united rather than competitors. Most in city commuter service was long ago discontinued due to the desires for abandonment by Class 1s of their day, along with agreed upon compromises with CTA for expansion. (Allegedly CTA had, at one time, agreed not to extend the L to O'Hare as the concession for extending to Jefferson Park.) Apparently, such competition off agreements were never negotiated with the I.C.
Ravenswood, now a top station on the entire system, survived due to light industry employment at the time and North Shore residents who wanted their cleaning ladies to be able to travel to their suburbs.
As for history of the era, I wish we still had Roy Benedict around to chime in. R.I.P. Roy.