Different from the early universe, heavy-ion collisions at very high energies do not reach statistical equilibrium, although thermal models explain many of their features. To account for the non-equilibrium nature analytically, a schematic model based on a Fokker-Planck equation is proposed and solved. The evolution of rapidity distributions from SIS via AGS and SPS to RHIC energies is discussed and compared with data, and the validity of the dissipation- fluctuation theorem (Einstein relation) is tested.