  The evolution of net-proton rapidity spectra with sqrt(s_NN) in heavy
relativistic systems is proposed as an indicator for local equilibration and
longitudinal expansion. In a Relativistic Diffusion Model, bell-shaped
distributions in central collisions at AGS energies and double-humped
nonequilibrium spectra at SPS show pronounced longitudinal collective expansion
when compared to the available data. The broad midrapidity valley recently
discovered at RHIC in central Au+Au collisions at sqrt(s_NN) = 200 GeV
indicates rapid local equilibration which is most likely due to deconfinement,
and fast longitudinal expansion of the locally equilibrated subsystem. A
prediction is made for Au+Au at sqrt(s_NN)= 62.4 GeV.
