Big shock for us when we took Caltrain and BART to SFO: our nice new credit cards to use in Europe (with the chip) were not accepted by either body's machines. Both credit card companies denied that it was true and claimed that their computers did not even show the attempt! But I promise you neither of our Visa cards worked. We had no other with us so had to use cash.

I would not count on a taxi anywhere. My son in Redwood City assured me that taxies were all over the Caltrain station for every train. Well, not when we got in. It was no big deal for us but if you have a flight to catch I would make a reservation with a cab company.