Per the Apple Pay website
(Link) it is accepted anywhere that takes contactless credit cards.
“Apple Pay is accepted at over 85 percent of retailers in the U.S., so you can likely use it wherever and however you want. If you’re not sure, just ask. Apple Pay works anywhere that takes contactless payments — from vending machines and grocery stores to taxis and subway stations.”
On edit: I only recently learned that the Apple Pay network is not required. When I added my BofA card to my wallet, BofA texted me a code then sent an email saying my CC was added to my iPhone wallet and could be used at any contactless station. I guess they added their transaction routing info to my phone in order to skip the need for the Apple Pay network.
Dave