Inclusivemeasurements of pion production in proton-proton collisions in the forward direction were undertaken at 400 and 600 MeV at the cooler synchrotron (COSY) at Forschungszentrum Julich using the Big Karl spectrograph. The high resolution in the π⁺ momentum ensured that there was an unambiguous separation of the pp → π⁺d/π⁺pn channels. Using these and earlier data, the ratio of the production cross sections could be followed through the D region and compared with the predictions of final state interaction theory. Deviations are strongly influenced by long-range terms in the production operator and the tensor force in the final pn system. These have been investigated in a realistic pp → π⁺d/π⁺pn calculation that includes S D channel coupling between the final nucleons. A semiquantitative understanding of the observed effects is achieved.