In this article a variational reduction method, how to handle the case of heterogenous
domains for the Transport equation, is presented. This method allows to get rid of the
restrictions on the size of time steps due to the thin parts of the domain. In the thin
part of the domain, only a differential problem, with respect to the space variable, is to
be approximated numerically. Numerical results are presented with a simple example. The
variational reduction method can be extended to thin domains multi-branching in 3
dimensions, which is a work in progress.