  Inclusive measurements of pion production in proton--proton collisions in the
forward direction were undertaken at 400 and 600 MeV at COSY using the Big Karl
spectrograph. The high resolution in the $\pi^+$ momentum ensured that there
was an unambiguous separation of the $pp\to {\pi}^+d/\pi^+pn$ channels. Using
these and earlier data, the ratio of the production cross sections could be
followed through the $\Delta$ 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 $pp\to\pi^+d/\pi^+pn$ calculation
that includes $S \rightleftharpoons D$ channel coupling between the final
nucleons. A semi-quantitative understanding of the observed effects is
achieved.
