Internal flaws in rail occur as a consequence of the accumulation of fatigue under repeated loading. Inspection of the rail to detect flaws before they progress to complete failure is thus very important. Advanced nondestructive inspection techniques are being pursued to improve rail safety, inspection productivity, and reduce train delays related with delayed action on the repair of non-critical rail flaws. Evaluations of various types of detector cars operating over known flaws in rail are being conducted at the Transportation Technology Center (TTC) on the Rail Defect Test Facility (RDTF), located on a gauntlet track. The facility was constructed in 1997 for a test program jointly sponsored by the Association of American Railroads and the Federal Railroad Administration (FRA) to provide member roads, the FRA and suppliers with a tool to assess detection technologies over known flaws in a controlled environment. Ongoing and future evaluations of new and emerging technologies are planned for the RDTF and will be conducted with funding from the jointly sponsored test program. Rail flaw data collected during testing will be made available to various program participants to assess improved analytical detection techniques. The test program will continue to sponsor the assessment of new and improved signal processing techniques to improve the reliability of signal interpretation by testing such concepts on the RDTF at TTC. This report focuses on the RDTF, results from evaluations performed on the RDTF, and future testing of improved and new rail flaw detection technologies planned at TTC. Additionally, a history of rail flaw detection is provided and a description of typical flaws found in North America, along with information on technology currently used for rail flaw detection.