Deterioration has been observed in concrete railroad ties, producing a loss of up to 40 mm at the railseat and leaving a rough surface formed mainly by aggregates. This deterioration occurs largely in cold and wet regions of western Canada and northern United States. The purpose of the present investigation was to deduce the processes responsible for this deterioration. Proposed mechanisms included abrasion, erosion, hydraulic pressure, freeze-thaw cycles, and chemical deterioration. Microstructural features associated with each mechanism were explored using laboratory concrete subjected to known deterioration processes or through computer simulations. Microstructural studies utilized scanning electron microscopy and x-ray diffraction. Based on the microstructural evidence, it was concluded that the railseat deterioration is produced by a combination of abrasion and freeze-thaw or hydraulic pressure.