This progress report presents the results of a fatigue investigation made on full-size 9 1/2-in-diam crankpins by the Association of American Railroads. The primary object of this study was to develop an improved crankpin-and-wheel assembly which will alleviate crankpin fatigue failures occurring in the wheel-fit portion in service. A total of 50 full-size crankpin-and-wheel assemblies were tested, using four different types of pins. All pins were of plain-carbon steel, normalized and tempered, having a carbon content of 0.52 per cent and manganese of 0.80 per cent. Large double-ended cantilever-type fatigue machines subjected the crankpin-and-wheel assemblies to rotating-hending stresses. The fatigue resistance of the conventional assembly, against the crankpin breaking off in the wheel fit, may be expressed as 100 per cent. The other three types of pins gave values of 136, 146, and over 200 per cent (Table 7). Even with this greatly improved breaking-off strength, shallow fatigue cracks were found to initiate in the wheel-fit portion of all four types of pins at bending stresses much below what was required to propagate the cracks to complete fracture.