This paper deals with an unloader queueing model in which N identical
trailers are unloaded by one or more unloaders. Theoretical solutions
are obtained with the assumption of negative exponential distributions
for trip times, unloading times, and finishing times. A cost model is
developed to determine the optimal number of trailers. In order to ev
aluate the model robustness, simulation models are developed for three
different distributions, exponential, second-order Erlang, and triang
ular. We demonstrate, through simulation results, that the unloader qu
eueing model is sufficiently robust to the variations of probability d
istributions.