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Ultrasonic techniques have been used for many years for the inspection of rail. These measurements can detect the presence of a wide variety of defects but there are practical difficulties with the technology. While large transverse cracks of the type likely to cause catastrophic failure can be detected, the large, critical defects can be masked by large numbers of small, surface defects along the length of the rail. It would be very useful to be able to determine reliably the largest defect size in a length of rail. Also alumino-thermic welds are difficult to inspect due to the typical defect orientation and the attenuation of the weld material. Guided waves provide a very attractive solution to these problems; they travel along the rail, for tens or hundreds of metres, and are partially reflected by any defects which are present. They are particularly sensitive to vertical defects and they are used at relatively low frequency so they are not significantly attenuated by weld material. With financial support from Network Rail, the authors have developed a practical inspection tool based on guided wave measurements. This paper describes the design of a pre-production prototype guided wave instrument suitable for site trials. Results obtained at a level crossing are presented that demonstrate the use of the system as a practical screening tool.