I once saw a transit advocate grumble in a YouTube video about the infrequent service in some urban-rail systems. Such grumbling may or may not be justified - there is no point in a lot of service without a lot of riders. Nevertheless, I decided to check on frequency of service in US commuter-rail systems.
ACE Transit (San Jose - Fremont - Livermore - Stockton) has four commute-time trains and no others: (4 trains/direction/dday). That is pretty much the rock bottom of commuter-rail service.
Caltrain: for San Francisco - San Jose, full service, and for San Jose - Gilroy, commute-time only (3 t/d/d). Caltrain SF-SJ off-peak headways: 1/2 hour on weekdays, 1 hour on weekends
Los Angeles Metrolink: mostly full but infrequent service. The most frequent train is the San Bernardino Line (18 t/d/d on weekdays, 8 t/d/d on weekends) and the least frequent train the Riverside Line (6 t/d/d weekdays only mostly peak time). The San Bernardino - Redlands line is more frequent, even if relatively short (headways: 1/2 - 1 hour on weekdays, 1 hour on weekends).
Chicago Metra:
- weekdays, weekends: Milwaukee, UP North, UP Northwest, Electric (the most frequent?), Rock Island, BNSF, Milwaukee West, UP West
- weekdays only: North Central (6c, 1r), Heritage (3c), Southwest (all day)
c = commute, r = reverse commute
No success with the South Shore Line, however.
DC:
- Virginia Railway Express: peak times only
- MARC:
- weekdays, weekends: Penn Line
- peak times only: Camden, Brunswick Lines
Philadelphia: SEPTA Regional Rail
- weekdays, weekends: all but Cynwyd
- weekdays only: Cynwyd (6 t/d/d)
Service headways are weekdays 1h, weekends 1h or 2h.
NYC and nearby:
- New Jersey Transit: weekdays, weekends: all, though the outer part of the Montclair-Boonton line seems to have the least frequent schedules
- Long Island RR: all weekdays, weekends: all, though the outer parts of the Montauk and Port Jefferson Lines are infrequent
- Metro-North: all weekdays, weekends: all
- Shore Line East: weekdays, weekends
Boston (MBTA): weekdays, weekends: all
I looked in GO Transit's site for schedules, without any success.