I was in a training class in Washington for 11 weeks from April-July 1976. The Metro opened in March 1976. As I recall, the original part was the Red Line from Union Station to Farragut North. They had people taking your money; this was before fare cards. I thought it was great. It was much more modern than the New York subway system which I had used when I lived there until I was 10. I am sad to know that it has had many repair problems. I’ll remember it as new and shiny.
Here are links about the history of Metro:
Happy birthday, Metro! Watch Metro’s evolution since 1976 in this slideshow
The Infuriating History of How Metro Got So Bad