  We study the driven Brownian motion of hard rods in a one-dimensional cosine
potential with an amplitude large compared to the thermal energy. In a closed
system, we find surprising features of the steady-state current in dependence
of the particle density. The form of the current-density relation changes
greatly with the particle size and can exhibit both a local maximum and
minimum. The changes are caused by an interplay of a barrier reduction,
blocking and exchange symmetry effect. The latter leads to a current equal to
that of non-interacting particles for a particle size commensurate with the
period length of the cosine potential. For an open system coupled to particle
reservoirs, we predict five different phases of non-equilibrium steady states
to occur. Our results show that the particle size can be of crucial importance
for non-equilibrium phase transitions in driven systems. Possible experiments
for demonstrating our findings are pointed out.
