A few mainline scheduled pax routes that come to mind, many of them from the pre-deregulation days when the CAB made airlines stop in several smaller cities to connect 2 large ones. In modern times, not only are these flights more time consuming than driving, but the airlines do not WANT local pax(i.e. terminating at the hub). A very short ticket will usually be a few hundred dollars, they don't want that seat going to your lazy ass when someone flying from Bulgaria or the Maldives might use it as a connection off an expensive ticket. Off the top of my head:
SFO/SJC-OAK - United, Western, PSA, Hughes Air West, Air California, ect. As late as the mid-late 90s, TWA did SJC-SFO-STL with MD-80s. Depart the 30s, right turn towards the 28 final into SFO, then left turn to join it. Less than 10 minutes in the air I've heard from people who did it.
MRY-SFO - United flew that route for decades, in the mid 90s the route had up to 6 mainline flights per day(727s, 737s, A320s) in addition to several UAX flights. From 1995-2001, United flew 2 daily 757-200s on the route with one being a RON. DC-8s were on the route in the late 70s doing DEN-MRY-SFO. Not sure how fast the jets did it, but when I did it on a Brasilia as a pax, flight times were 19 mins and 21 mins. Other carriers like Pacific Express, PSA, and Air California also flew from MRY to Bay Area airports.
SFO-SMF - PSA, United, Hughes Airwest, ect. United kept mainline metal on the route into the 90s and recently brought 737-800s back on the route. Back in the day, they called it a 20 minute flight(including taxi). Now United calls it 45 mins to an hour. Good ol' congested skies.
OGG-KOA - ATA and Rich International flew L-1011s on scheduled flights between the two in the 90s.
ONT-LAX- Air California with 737-200s in addition to dozens of commuters. FedEx and UPS still have flights between SNA/ONT/LAX/BUR today with A300s and 757s/767s.
PSP-ONT - Air California with 737-200s for a short while.
PHX-TUS - America West and US Airways operated this route mainline into the late 2000s(got to do it as a pax in an A319 in 2009 or 2010), many airlines like United and Delta operated the route at different times, Delta with 757s at one point.
MSP-RST - Northwest once had several DC-9s a day on this route, close to a dozen at one point.
SAN-LAX - Delta and TWA once had L-1011s on this route, I think PSA did too. At least a dozen airlines have done it with mainline metal.
DEN-COS - United had cheap flights with DC-10s on the route in the 90s to try and run Western Pacific into the ground. 737s were on the route for decades as well(UA535 crashed on that route).
Pacific Express operated all its routes in the early/mid 80s with Bac 1-11s and 737-200s(those mostly did NorCal/SoCal flying though not the smaller cities). This is their 1983 route map(yes, SCK-MOD in a mainline jet!):
