  In this paper we develop the theory of train track maps on graphs of groups.
Expanding a definition of Bass, we define a notion of a map of a graph of
groups, and of a homotopy equivalence. We prove that under one of two technical
hypotheses, any homotopy equivalence of a graph of groups may be represented by
a relative train track map. The first applies in particular to graphs of groups
with finite edge groups, while the second applies in particular to certain
generalized Baumslag-Solitar groups.
