Paying for Transportation Tickets Using a US credit card

For the ticket automats at the Copenhagen airport, no (two Americans ahead of me in line were having trouble). I'm not sure about the counter service. The machines accept Danish kroner, although to complicate matters, the "ATMs" at the Copenhagen airport are actually automatic money exchanges, with all the associated mark-ups. I don't know about the automats for the Flytoget tickets in Norway. For a T Bahn ticket, if in doubt, you can purchase an Oslo pass from the TI, which includes unlimited Tbahn use.

Our experience two years ago for that area was that we had to buy at the window and pay a service charge for using the window. We use our Am credit cards.