  In this paper we consider a model of particles jumping on a row of cells,
called in physics the one dimensional totally asymmetric exclusion process
(TASEP). More precisely we deal with the TASEP with open or periodic boundary
conditions and with two or three types of particles. From the point of view of
combinatorics a remarkable feature of this Markov chain is that it involves
Catalan numbers in several entries of its stationary distribution. We give a
combinatorial interpretation and a simple proof of these observations. In doing
this we reveal a second row of cells, which is used by particles to travel
backward. As a byproduct we also obtain an interpretation of the occurrence of
the Brownian excursion in the description of the density of particles on a long
row of cells.
