Abstract The most common rail defect encountered in continuously welded rail is known as the detail fracture. The U.S. Department of Transportation, Federal Railroad Administration has sponsored and managed research over the past several decades to understand the structural integrity of rail in general, and the fatigue crack growth behavior of detail fractures in particular. Control of rail integrity and defect growth is conducted via periodic rail tests (i.e. inspections) to ensure that rail defects do not become large enough to cause rail failure. Moreover, federal regulations have been codified to establish a maximum interval between rail inspections based on the results of government-sponsored research. Over the past several decades, however, rail manufacturing has evolved and improved, particularly the head-hardening process to improve wear resistance. Propagation life of railroad rail was examined in previous research using fatigue crack growth data for non-head-hardened rail. Recently Thornton-Tomasetti conducted research, sponsored by FRA, to examine the fatigue crack growth behavior of modern rail steels (i.e. railroad rails with head-hardening). The initial results of the more recent research effort were reported in the 2019 Joint Rail Conference. In this paper, fatigue crack growth rate data generated for head-hardened rail are used to examine the fatigue crack growth life of detail fractures under nominal revenue service conditions. Moreover, this paper applies a probabilistic approach to estimate rail life to account for the inherent variability or scatter typically observed in fatigue crack growth rate data. Regression methods are employed to derive the parameters for the Walker crack growth rate equation, which are subsequently treated as correlated, multivariate, and normally distributed random variables. Data from four different rail steels are used in the regression analyses, which are referred to as: Advanced Head Hardened (AHH), Head Hardened (HH), Standard Strength (SS), and Colorado Fuel and Iron (CF&I). Monte Carlo simulations of fatigue growth of detail fractures are carried out to estimate fatigue life distributions for each of the different rails. The results from these four rail steels are compared to those based on the previous research for non-head-hardened rails. Implications of these comparisons on determining rail testing intervals are discussed.