I always take a couple extra passport type pictures and buy the travelcard at my first tube stop. It is very easy to do. I get the one zone since I usually don't plan to go out of it or will take the train if I'm going to Kew to the Public Records Office. I know you can buy a one day and maybe four day and 7 day. I am usually there for a week and get the week card. I don't think it is any cheaper than getting a ticket each time you get on, but it sure saves time and long lines. You can get off and on any bus or tube so it is very convenient. I don't know about the card bought here in the US so I can't help with that. If you don't take extra pictures with you, there were photo booths available in a number of tube stops so you could buy the card before your first trip.

Beth Sloan Las Vegas