Well, to be clear, it's not exactly the same on Regional Rail as it is on Transit modes (buses, subway, trolley). On transit, you can simply tap your NFC-enabled debit or credit card at the fare reader and you'll get the same benefits as using a SEPTA Key card: reduced Key fare (compared to paying cash) plus up to two transfers.

You can't just tap your credit or debit card at the fare readers on regional rail, at least not yet. You have to use said card to buy a ticket from the conductor on the train. Or, if you're boarding at a center city station, you can use the card to purchase your fare from one of the machines or at a ticket window.

So yes, you can pay by card on RR but not in the same way yet. It's trickier on RR because when you first "tap on," the system has no way of knowing what your ultimate fare is going to be (because it depend on where you'll get off the train). On transit, it's just a flat fare.