The National Park Service is continuing to convert dozens of its sites across the country to cashless payments only, drawing complaints and, now, a lawsuit. Starting in June last year, visitors to Rocky Mountain National Park in Colorado were told that they could not use cash to enter the park or use its campgrounds. The negative reactions were swift, with visitors raising privacy concerns and expressing confusion about why the American dollar would not be welcome in the U.S. parks system. Now these complaints are the subject of a lawsuit filed on March 6 in U.S. District Court in the District of Columbia, asserting that the service's policies violate federal law defining cash as "legal tender" and the visitors' "lawful right to pay in cash" at national sites, including those without bank accounts or cards or those who simply prefer to pay cash. In addition to the park service, its director, Charles F. Sams, III, and the Department of the Interior were named as defendants. One of the three plaintiffs, Toby Stover, a New York woman, drove to Hyde Park, N.Y., in January. She ... was not allowed to enter after trying to pay $10 in cash, the filing said. The plaintiffs' lawyer, Ray L. Flores, II, said in an emailed reply to questions that the legal action is being financially backed by the Children's Health Defense. Mr. Flores said in the email that cashless policies were "a key component " if not the linchpin " of the surveillance state."