The buildup of thermally induced compressive stress in continuously welded railroad rail [1,2] can result in track buckling (lateral displacement of the track over a distance of approximately 100 feet [3]) which can cause derailment of a passing train. Therefore, a direct stress measurement approach utilizing a probe which could be scanned continuously along the rail would be very valuable for surveying the state of rail stress to determine if a potentially unsafe condition exists. In the research described in this paper, the nonlinear harmonics (NLH) method was investigated as an approach for measuring stress in a railroad rail. Laboratory measurements were taken while the probe was scanned along the web of a 3-foot-long section of rail subjected to compressive stress in a loading machine. Approaches for reducing the effect of material property variations by using stress-induced anisotropy and spatial averaging of the data were also investigated and shown to be very promising.