  In this paper, we study the concept of approximate controllability of
retarded network systems of neutral type. On one hand, we reformulate such
systems as free-delay boundary control systems on product spaces. On the other
hand, we use the rich theory of infinite-dimensional linear systems to derive
necessary and sufficient conditions for approximate controllability. Moreover,
we propose a rank condition for which we can easily verify the conditions of
controllability. Our approach is mainly based on the feedback theory of regular
linear systems in the Salamon-Weiss sense.
