  We study a two-species bidirectional exclusion process, and a single species
variant, which is motivated by the motion of organelles and vesicles along
microtubules. Specifically, we are interested in the clustering of the
particles and appearance of a single large cluster as the ratio $Q$ of the
translation to switching rates is varied. We find that, although for a finite
system, there is a clustering phenomenon in which the probability of finding a
single large cluster changes from being negligible to having finite values, the
phenomenon shifts to larger $Q$ values as the system size is increased. This
suggests that the observed clustering is not a true (nonequilibrium) transition
in the thermodynamic sense but rather a finite-size effect.
