The Federal Railroad Administration, part of the Department of Transportation, was created by the Department of Transportation Act of 1966. Its purposes include both the writing and enforcing of rail safety regulations, the investigation of railroad accidents, research and development for improved railroad safety and in support of national rail transportation policy, and to act as the conduit through which federal support of rail transportation is provided, including the support of Amtrak.
In many ways, the FRA is analogous to the Federal Aviation Administration.
The FRA owns and operates a test track at Pueblo, Colorado for the testing of rail equipment under controlled conditions. It's used among other things for testing new Amtrak equipment, testing new urban rapid transit cars, by the governments of the United States, Canada and Japan, and by many private corporations.
Of the FRA's responsibilities, the most visible is probably its safety mission. The FRA is responsible for setting safety regulations for the industry, and has broad regulatory powers to force railroads to change their operating practices, alter equipment, even to stop operation until problems are fixed. The FRA investigates all accidents and has authority to compel the release of information and the cooperation of railroads, employees and individuals.
The FRA is particularly concerned with grade crossing safety issues, since grade crossing accidents cause over half of all railroad related deaths and an even larger proportion of deaths among the general public. In coordination with railroads, state agencies and organisations such as Operation Lifesaver, the FRA attempts to educate about railroad grade-crossing and trespass hazards, to implement safety measures to increase safety at those crossings, and to research how to reduce the casualties in future.
Some facts from the FRA's own web site, http://www.fra.dot.gov/