Thanks to jennakalkwarf who posted this tip on Sweden discussion board, I was easily able in English to buy train tickets on the high speed (non stop) train from Copenhagen to Stockholm here in the US. I kept trying the Danish rail site listed in Rick's book under "connections" in Copenhagen, but I could not pull up the non stop trains -- and the site was hard to understand even when I chose English as the language. But the Swedish site for the same train system was easy to navigate and understand. Furthermore, my US credit card was easily accepted from here in the US. The Swedish site for this train is: https://www.sj.se/en/home.html#/
Buying directly from the train company, I was able to avoid the 15-25% markup that middlemen such as RailEurope charge.
Thanks again jennakalkwarf!