The non-uniform longitudinal flow model (NUFM) proposed recently is extended to include also the transverse flow. The resulting longitudinally non-uniform collective expansion model (NUCEM) is applied to the calculation of rapidity distribution of kaons, lambdas and protons in relativistic heavy ion collisions at CERN-SPS energies. The model results are compared with the 200 A GeV/ c S-S and 158 A GeV/ c Pb-Pb collision data. The central dips observed in experiments are reproduced in a natural way. ;It is found that the depth of the central dip depends on the magnitude of the parameter e and t...
