The Upshot has a short article on rail trails and how they got their start.
On this week in 1964, Interior Secretary Stewart Udall called for more bicycle paths in the United States. He would go on to push for enactment of the National Trails System Act of 1968, which established a nationwide program to build recreational trails, many of them on rights-of-way freed up by the decline of the railroad industry. Another federal law made more money for bicycling available after 1973. By 1987, when the Rails-to-Trails Conservancy started tracking mileage, there were 1,100 miles of rail trails; more than 21,000 miles exist today.
And it features this fantastic photo of Secretary Udall and members of congress riding bikes along what looks like Jefferson Street SW, south of the Mall. But I can't be sure. Can anyone place this more accurately?
Look at Udall in his hat. What a hipster he was. By the way, he's the father of current Senator Tom Udall and uncle to Senator Mark Udall.