A method is developed and validated to reconstruct the full-length longitudinal residual-stress field in railway rails from measurements on a short, partially relaxed specimen. The approach represents the stress on the rail cross-section using a set of admissible (self-equilibrated) basis functions, generated from out-of-plane bending mode shapes of a thin-plate model of the cross-section. Stress relaxation due to specimen extraction is simulated for each basis function, enabling the measured relaxed field to be decomposed and mapped back to the corresponding initial field. Numerical tests and contour-method measurements on a used rail show that the method can recover the main features of the pre-extraction stress state and capture the dominant large-scale relaxation behavior, while remaining reasonably robust to smaller-scale measurement artefacts. The framework is intended to make residual-stress assessment more practical when only short specimens can be extracted, and it is expected to transfer to other continuously processed components.