In my experience in Chicago and Boston there's a big difference between scheduled service and frequency service.
The commuter rail systems have a timetable that riders must be aware of. Missing a train can result in a long delay, so you plan to arrive at the station based on the timetable. They are basically intercity trains with frequent stations. They also share typically track with other trains, including freight. That can result in delays on the schedule due to interference from non-commuter trains.
The rapid transit systems have no preset timetable. You show up at the station and know that approximately every 10 minutes (or whatever) a train will show up. Busier stations have screens to say when the next train is arriving, but that's new in the last decade. It doesn't change the fact that when you use them you just show up whenever you can get there. Delays are usually due to station congestion slowing the trains ahead of you, but since there isn't a fixed schedule it doesn't throw off a larger timetable.
There really isn't a fixed schedule, or riders are simply not aware of it because the frequency is so high they just go to the station and don't bother to find out when the next train shows up? For instance, when riding the New York subway, most commuters will probably not bother finding out what the schedule is (I certainly didn't), but there definitely is one, and by night it can suddenly become useful to look it up because the frequency of trains is lower.
My point is that there are likely timetables for both forms of public transportation. The difference solely consists of the frequency and of the question whether riders will bother looking up the timetable, but these are very subjective criteria. One person may just go and find it okay to wait for 15 minutes, another may look up the timetable. It's hard to distinguish between various types of public transportation on the basis of this criterium.
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