Abstract: An apparatus for determining a coefficient of friction on a surface of objects such as rails. In one embodiment, the apparatus comprises a vertical sensor assembly for determining the coefficient of friction of a top surface of a rail. A different embodiment comprises a lateral sensor assembly for determining a coefficient of friction of a gage face of a rail. The sensor assemblies may include a shoe. The sensor assemblies may include a load cell. Other embodiments include various combinations of lateral sensor assemblies and vertical sensor assemblies. Structure for mounting the assemblies to a vehicle may also be provided.